diff --git a/lib/client.js b/lib/client.js
index 2fec533faf1..b83863e296b 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -765,11 +765,14 @@ class Parser {
     }
 
     const key = this.headers[len - 2]
-    if (key.length === 10 && key.toString().toLowerCase() === 'keep-alive') {
-      this.keepAlive += buf.toString()
-    } else if (key.length === 10 && key.toString().toLowerCase() === 'connection') {
-      this.connection += buf.toString()
-    } else if (key.length === 14 && key.toString().toLowerCase() === 'content-length') {
+    if (key.length === 10) {
+      const headerName = util.bufferToLowerCasedHeaderName(key)
+      if (headerName === 'keep-alive') {
+        this.keepAlive += buf.toString()
+      } else if (headerName === 'connection') {
+        this.connection += buf.toString()
+      }
+    } else if (key.length === 14 && util.bufferToLowerCasedHeaderName(key) === 'content-length') {
       this.contentLength += buf.toString()
     }
 
diff --git a/lib/core/util.js b/lib/core/util.js
index 49d1c9938ed..6c2e1e55d21 100644
--- a/lib/core/util.js
+++ b/lib/core/util.js
@@ -226,7 +226,14 @@ function parseKeepAliveTimeout (val) {
 function headerNameToString (value) {
   return typeof value === 'string'
     ? headerNameLowerCasedRecord[value] ?? value.toLowerCase()
-    : tree.lookup(value) ?? value.toString().toLowerCase()
+    : tree.lookup(value) ?? value.toString('latin1').toLowerCase()
+}
+
+/**
+ * @param {Buffer} value
+ */
+function bufferToLowerCasedHeaderName (value) {
+  return tree.lookup(value) ?? value.toString('latin1').toLowerCase()
 }
 
 /**
@@ -523,6 +530,7 @@ module.exports = {
   isAsyncIterable,
   isDestroyed,
   headerNameToString,
+  bufferToLowerCasedHeaderName,
   parseRawHeaders,
   parseHeaders,
   parseKeepAliveTimeout,
diff --git a/lib/fetch/body.js b/lib/fetch/body.js
index 2a8b38f7415..6202887289a 100644
--- a/lib/fetch/body.js
+++ b/lib/fetch/body.js
@@ -374,7 +374,7 @@ function bodyMixinMethods (instance) {
       // If mimeType’s essence is "multipart/form-data", then:
       if (/multipart\/form-data/.test(contentType)) {
         const headers = {}
-        for (const [key, value] of this.headers) headers[key.toLowerCase()] = value
+        for (const [key, value] of this.headers) headers[key] = value
 
         const responseFormData = new FormData()
 
diff --git a/lib/fetch/headers.js b/lib/fetch/headers.js
index 2f1c0be5a47..883d7637cbf 100644
--- a/lib/fetch/headers.js
+++ b/lib/fetch/headers.js
@@ -135,13 +135,22 @@ class HeadersList {
     }
   }
 
-  // https://fetch.spec.whatwg.org/#header-list-contains
+  /**
+   * @see https://fetch.spec.whatwg.org/#header-list-contains
+   * @param {string} name
+   */
   contains (name) {
     // A header list list contains a header name name if list
     // contains a header whose name is a byte-case-insensitive
     // match for name.
-    name = name.toLowerCase()
 
+    return this[kHeadersMap].has(name.toLowerCase())
+  }
+
+  /**
+   * @param {string} name
+   */
+  lowerCaseContains (name) {
     return this[kHeadersMap].has(name)
   }
 
@@ -151,7 +160,11 @@ class HeadersList {
     this.cookies = null
   }
 
-  // https://fetch.spec.whatwg.org/#concept-header-list-append
+  /**
+   * @see https://fetch.spec.whatwg.org/#concept-header-list-append
+   * @param {string} name
+   * @param {string} value
+   */
   append (name, value) {
     this[kHeadersSortedMap] = null
 
@@ -172,12 +185,42 @@ class HeadersList {
     }
 
     if (lowercaseName === 'set-cookie') {
-      this.cookies ??= []
-      this.cookies.push(value)
+      (this.cookies ??= []).push(value)
+    }
+  }
+
+  /**
+   * @param {string} name
+   * @param {string} value
+   */
+  lowerCaseAppend (name, value) {
+    this[kHeadersSortedMap] = null
+
+    // 1. If list contains name, then set name to the first such
+    //    header’s name.
+    const exists = this[kHeadersMap].get(name)
+
+    // 2. Append (name, value) to list.
+    if (exists) {
+      const delimiter = name === 'cookie' ? '; ' : ', '
+      this[kHeadersMap].set(name, {
+        name: exists.name,
+        value: `${exists.value}${delimiter}${value}`
+      })
+    } else {
+      this[kHeadersMap].set(name, { name, value })
+    }
+
+    if (name === 'set-cookie') {
+      (this.cookies ??= []).push(value)
     }
   }
 
-  // https://fetch.spec.whatwg.org/#concept-header-list-set
+  /**
+   * @see https://fetch.spec.whatwg.org/#concept-header-list-set
+   * @param {string} name
+   * @param {string} value
+   */
   set (name, value) {
     this[kHeadersSortedMap] = null
     const lowercaseName = name.toLowerCase()
@@ -193,11 +236,35 @@ class HeadersList {
     this[kHeadersMap].set(lowercaseName, { name, value })
   }
 
-  // https://fetch.spec.whatwg.org/#concept-header-list-delete
+  /**
+   * @param {string} name
+   * @param {string} value
+   */
+  lowerCaseSet (name, value) {
+    if (name === 'set-cookie') {
+      this.cookies = [value]
+    }
+
+    // 1. If list contains name, then set the value of
+    //    the first such header to value and remove the
+    //    others.
+    // 2. Otherwise, append header (name, value) to list.
+    this[kHeadersMap].set(name, { name, value })
+  }
+
+  /**
+   * @see https://fetch.spec.whatwg.org/#concept-header-list-delete
+   * @param {string} name
+   */
   delete (name) {
-    this[kHeadersSortedMap] = null
+    return this.lowerCaseDelete(name.toLowerCase())
+  }
 
-    name = name.toLowerCase()
+  /**
+   * @param {string} name
+   */
+  lowerCaseDelete (name) {
+    this[kHeadersSortedMap] = null
 
     if (name === 'set-cookie') {
       this.cookies = null
@@ -206,7 +273,11 @@ class HeadersList {
     this[kHeadersMap].delete(name)
   }
 
-  // https://fetch.spec.whatwg.org/#concept-header-list-get
+  /**
+   * @see https://fetch.spec.whatwg.org/#concept-header-list-get
+   * @param {string} name
+   * @returns {string | null}
+   */
   get (name) {
     const value = this[kHeadersMap].get(name.toLowerCase())
 
@@ -217,6 +288,14 @@ class HeadersList {
     return value === undefined ? null : value.value
   }
 
+  /**
+   * @param {string} name
+   * @returns {string | null}
+   */
+  lowerCaseGet (name) {
+    return this[kHeadersMap].get(name)?.value ?? null
+  }
+
   * [Symbol.iterator] () {
     // use the lowercased name
     for (const [name, { value }] of this[kHeadersMap]) {
diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index b3b6e71eb79..0e5fc35a13d 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -57,7 +57,7 @@ const {
 const { kHeadersList, kConstruct } = require('../core/symbols')
 const EE = require('events')
 const { Readable, pipeline } = require('stream')
-const { addAbortListener, isErrored, isReadable, nodeMajor, nodeMinor } = require('../core/util')
+const { addAbortListener, isErrored, isReadable, nodeMajor, nodeMinor, bufferToLowerCasedHeaderName } = require('../core/util')
 const { dataURLProcessor, serializeAMimeType, parseMIMEType } = require('./dataURL')
 const { getGlobalDispatcher } = require('../global')
 const { webidl } = require('./webidl')
@@ -475,7 +475,7 @@ function fetching ({
   }
 
   // 12. If request’s header list does not contain `Accept`, then:
-  if (!request.headersList.contains('accept')) {
+  if (!request.headersList.lowerCaseContains('accept')) {
     // 1. Let value be `*/*`.
     const value = '*/*'
 
@@ -492,14 +492,14 @@ function fetching ({
     // TODO
 
     // 3. Append `Accept`/value to request’s header list.
-    request.headersList.append('accept', value)
+    request.headersList.lowerCaseAppend('accept', value)
   }
 
   // 13. If request’s header list does not contain `Accept-Language`, then
   // user agents should append `Accept-Language`/an appropriate value to
   // request’s header list.
-  if (!request.headersList.contains('accept-language')) {
-    request.headersList.append('accept-language', '*')
+  if (!request.headersList.lowerCaseContains('accept-language')) {
+    request.headersList.lowerCaseAppend('accept-language', '*')
   }
 
   // 14. If request’s priority is null, then use request’s initiator and
@@ -718,7 +718,7 @@ async function mainFetch (fetchParams, recursive = false) {
     response.type === 'opaque' &&
     internalResponse.status === 206 &&
     internalResponse.rangeRequested &&
-    !request.headers.contains('range')
+    !request.headers.lowerCaseContains('range')
   ) {
     response = internalResponse = makeNetworkError()
   }
@@ -840,7 +840,7 @@ function schemeFetch (fetchParams) {
 
       // 8. If request’s header list does not contain `Range`:
       // 9. Otherwise:
-      if (!request.headersList.contains('range')) {
+      if (!request.headersList.lowerCaseContains('range')) {
         // 1. Let bodyWithType be the result of safely extracting blob.
         // Note: in the FileAPI a blob "object" is a Blob *or* a MediaSource.
         // In node, this can only ever be a Blob. Therefore we can safely
@@ -854,14 +854,14 @@ function schemeFetch (fetchParams) {
         response.body = bodyWithType[0]
 
         // 4. Set response’s header list to « (`Content-Length`, serializedFullLength), (`Content-Type`, type) ».
-        response.headersList.set('content-length', serializedFullLength)
-        response.headersList.set('content-type', type)
+        response.headersList.lowerCaseSet('content-length', serializedFullLength)
+        response.headersList.lowerCaseSet('content-type', type)
       } else {
         // 1. Set response’s range-requested flag.
         response.rangeRequested = true
 
         // 2. Let rangeHeader be the result of getting `Range` from request’s header list.
-        const rangeHeader = request.headersList.get('range')
+        const rangeHeader = request.headersList.lowerCaseGet('range')
 
         // 3. Let rangeValue be the result of parsing a single range header value given rangeHeader and true.
         const rangeValue = simpleRangeHeaderValue(rangeHeader, true)
@@ -921,9 +921,9 @@ function schemeFetch (fetchParams) {
 
         // 15. Set response’s header list to « (`Content-Length`, serializedSlicedLength),
         //     (`Content-Type`, type), (`Content-Range`, contentRange) ».
-        response.headersList.set('content-length', serializedSlicedLength)
-        response.headersList.set('content-type', type)
-        response.headersList.set('content-range', contentRange)
+        response.headersList.lowerCaseSet('content-length', serializedSlicedLength)
+        response.headersList.lowerCaseSet('content-type', type)
+        response.headersList.lowerCaseSet('content-range', contentRange)
       }
 
       // 10. Return response.
@@ -1040,7 +1040,7 @@ function fetchFinale (fetchParams, response) {
         responseStatus = response.status
 
         // 2. Let mimeType be the result of extracting a MIME type from response’s header list.
-        const mimeType = parseMIMEType(response.headersList.get('content-type')) // TODO: fix
+        const mimeType = parseMIMEType(response.headersList.lowerCaseGet('content-type')) // TODO: fix
 
         // 3. If mimeType is not failure, then set bodyInfo’s content type to the result of minimizing a supported MIME type given mimeType.
         if (mimeType !== 'failure') {
@@ -1336,11 +1336,11 @@ function httpRedirectFetch (fetchParams, response) {
   //     delete headerName from request’s header list.
   if (!sameOrigin(requestCurrentURL(request), locationURL)) {
     // https://fetch.spec.whatwg.org/#cors-non-wildcard-request-header-name
-    request.headersList.delete('authorization')
+    request.headersList.lowerCaseDelete('authorization')
 
     // "Cookie" and "Host" are forbidden request-headers, which undici doesn't implement.
-    request.headersList.delete('cookie')
-    request.headersList.delete('host')
+    request.headersList.lowerCaseDelete('cookie')
+    request.headersList.lowerCaseDelete('host')
   }
 
   // 14. If request’s body is non-null, then set request’s body to the first return
@@ -1456,7 +1456,7 @@ async function httpNetworkOrCacheFetch (
   //    `Content-Length`/contentLengthHeaderValue to httpRequest’s header
   //    list.
   if (contentLengthHeaderValue != null) {
-    httpRequest.headersList.append('content-length', contentLengthHeaderValue)
+    httpRequest.headersList.lowerCaseAppend('content-length', contentLengthHeaderValue)
   }
 
   //    9. If contentLengthHeaderValue is non-null, then append (`Content-Length`,
@@ -1472,7 +1472,7 @@ async function httpNetworkOrCacheFetch (
   //    `Referer`/httpRequest’s referrer, serialized and isomorphic encoded,
   //     to httpRequest’s header list.
   if (httpRequest.referrer instanceof URL) {
-    httpRequest.headersList.append('referer', isomorphicEncode(httpRequest.referrer.href))
+    httpRequest.headersList.lowerCaseAppend('referer', isomorphicEncode(httpRequest.referrer.href))
   }
 
   //    12. Append a request `Origin` header for httpRequest.
@@ -1484,8 +1484,8 @@ async function httpNetworkOrCacheFetch (
   //    14. If httpRequest’s header list does not contain `User-Agent`, then
   //    user agents should append `User-Agent`/default `User-Agent` value to
   //    httpRequest’s header list.
-  if (!httpRequest.headersList.contains('user-agent')) {
-    httpRequest.headersList.append('user-agent', typeof esbuildDetection === 'undefined' ? 'undici' : 'node')
+  if (!httpRequest.headersList.lowerCaseContains('user-agent')) {
+    httpRequest.headersList.lowerCaseAppend('user-agent', typeof esbuildDetection === 'undefined' ? 'undici' : 'node')
   }
 
   //    15. If httpRequest’s cache mode is "default" and httpRequest’s header
@@ -1494,11 +1494,11 @@ async function httpNetworkOrCacheFetch (
   //    httpRequest’s cache mode to "no-store".
   if (
     httpRequest.cache === 'default' &&
-    (httpRequest.headersList.contains('if-modified-since') ||
-      httpRequest.headersList.contains('if-none-match') ||
-      httpRequest.headersList.contains('if-unmodified-since') ||
-      httpRequest.headersList.contains('if-match') ||
-      httpRequest.headersList.contains('if-range'))
+    (httpRequest.headersList.lowerCaseContains('if-modified-since') ||
+      httpRequest.headersList.lowerCaseContains('if-none-match') ||
+      httpRequest.headersList.lowerCaseContains('if-unmodified-since') ||
+      httpRequest.headersList.lowerCaseContains('if-match') ||
+      httpRequest.headersList.lowerCaseContains('if-range'))
   ) {
     httpRequest.cache = 'no-store'
   }
@@ -1510,44 +1510,44 @@ async function httpNetworkOrCacheFetch (
   if (
     httpRequest.cache === 'no-cache' &&
     !httpRequest.preventNoCacheCacheControlHeaderModification &&
-    !httpRequest.headersList.contains('cache-control')
+    !httpRequest.headersList.lowerCaseContains('cache-control')
   ) {
-    httpRequest.headersList.append('cache-control', 'max-age=0')
+    httpRequest.headersList.lowerCaseAppend('cache-control', 'max-age=0')
   }
 
   //    17. If httpRequest’s cache mode is "no-store" or "reload", then:
   if (httpRequest.cache === 'no-store' || httpRequest.cache === 'reload') {
     // 1. If httpRequest’s header list does not contain `Pragma`, then append
     // `Pragma`/`no-cache` to httpRequest’s header list.
-    if (!httpRequest.headersList.contains('pragma')) {
-      httpRequest.headersList.append('pragma', 'no-cache')
+    if (!httpRequest.headersList.lowerCaseContains('pragma')) {
+      httpRequest.headersList.lowerCaseAppend('pragma', 'no-cache')
     }
 
     // 2. If httpRequest’s header list does not contain `Cache-Control`,
     // then append `Cache-Control`/`no-cache` to httpRequest’s header list.
-    if (!httpRequest.headersList.contains('cache-control')) {
-      httpRequest.headersList.append('cache-control', 'no-cache')
+    if (!httpRequest.headersList.lowerCaseContains('cache-control')) {
+      httpRequest.headersList.lowerCaseAppend('cache-control', 'no-cache')
     }
   }
 
   //    18. If httpRequest’s header list contains `Range`, then append
   //    `Accept-Encoding`/`identity` to httpRequest’s header list.
-  if (httpRequest.headersList.contains('range')) {
-    httpRequest.headersList.append('accept-encoding', 'identity')
+  if (httpRequest.headersList.lowerCaseContains('range')) {
+    httpRequest.headersList.lowerCaseAppend('accept-encoding', 'identity')
   }
 
   //    19. Modify httpRequest’s header list per HTTP. Do not append a given
   //    header if httpRequest’s header list contains that header’s name.
   //    TODO: https://github.com/whatwg/fetch/issues/1285#issuecomment-896560129
-  if (!httpRequest.headersList.contains('accept-encoding')) {
+  if (!httpRequest.headersList.lowerCaseContains('accept-encoding')) {
     if (urlHasHttpsScheme(requestCurrentURL(httpRequest))) {
-      httpRequest.headersList.append('accept-encoding', 'br, gzip, deflate')
+      httpRequest.headersList.lowerCaseAppend('accept-encoding', 'br, gzip, deflate')
     } else {
-      httpRequest.headersList.append('accept-encoding', 'gzip, deflate')
+      httpRequest.headersList.lowerCaseAppend('accept-encoding', 'gzip, deflate')
     }
   }
 
-  httpRequest.headersList.delete('host')
+  httpRequest.headersList.lowerCaseDelete('host')
 
   //    20. If includeCredentials is true, then:
   if (includeCredentials) {
@@ -1630,7 +1630,7 @@ async function httpNetworkOrCacheFetch (
 
   // 12. If httpRequest’s header list contains `Range`, then set response’s
   // range-requested flag.
-  if (httpRequest.headersList.contains('range')) {
+  if (httpRequest.headersList.lowerCaseContains('range')) {
     response.rangeRequested = true
   }
 
@@ -2121,15 +2121,15 @@ async function httpNetworkFetch (
           // We distinguish between them and iterate accordingly
           if (Array.isArray(rawHeaders)) {
             for (let i = 0; i < rawHeaders.length; i += 2) {
-              headersList.append(rawHeaders[i].toString('latin1'), rawHeaders[i + 1].toString('latin1'))
+              headersList.lowerCaseAppend(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'))
             }
-            const contentEncoding = headersList.get('content-encoding')
+            const contentEncoding = headersList.lowerCaseGet('content-encoding')
             if (contentEncoding) {
               // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
               // "All content-coding values are case-insensitive..."
               codings = contentEncoding.toLowerCase().split(',').map((x) => x.trim())
             }
-            location = headersList.get('location')
+            location = headersList.lowerCaseGet('location')
           } else {
             const keys = Object.keys(rawHeaders)
             for (let i = 0; i < keys.length; ++i) {
@@ -2243,7 +2243,7 @@ async function httpNetworkFetch (
           const headersList = new HeadersList()
 
           for (let i = 0; i < rawHeaders.length; i += 2) {
-            headersList.append(rawHeaders[i].toString('latin1'), rawHeaders[i + 1].toString('latin1'))
+            headersList.lowerCaseAppend(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'))
           }
 
           resolve({
diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index b0b0a2ce543..54ef3595416 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -126,7 +126,7 @@ class Response {
     const value = isomorphicEncode(URLSerializer(parsedURL))
 
     // 7. Append `Location`/value to responseObject’s response’s header list.
-    responseObject[kState].headersList.append('location', value)
+    responseObject[kState].headersList.lowerCaseAppend('location', value)
 
     // 8. Return responseObject.
     return responseObject
@@ -496,8 +496,8 @@ function initializeResponse (response, init, body) {
 
     // 3. If body's type is non-null and response's header list does not contain
     //    `Content-Type`, then append (`Content-Type`, body's type) to response's header list.
-    if (body.type != null && !response[kState].headersList.contains('Content-Type')) {
-      response[kState].headersList.append('content-type', body.type)
+    if (body.type != null && !response[kState].headersList.lowerCaseContains('content-type')) {
+      response[kState].headersList.lowerCaseAppend('content-type', body.type)
     }
   }
 }
diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index 9843f97444b..13c4b468cc6 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -35,7 +35,7 @@ function responseLocationURL (response, requestFragment) {
 
   // 2. Let location be the result of extracting header list values given
   // `Location` and response’s header list.
-  let location = response.headersList.get('location')
+  let location = response.headersList.lowerCaseGet('location')
 
   // 3. If location is a header value, then set location to the result of
   //    parsing location with response’s URL.
@@ -153,7 +153,7 @@ function setRequestReferrerPolicyOnRedirect (request, actualResponse) {
   // 2. Let policy be the empty string.
   // 3. For each token in policy-tokens, if token is a referrer policy and token is not the empty string, then set policy to token.
   // 4. Return policy.
-  const policyHeader = (headersList.get('referrer-policy') ?? '').split(',')
+  const policyHeader = (headersList.lowerCaseGet('referrer-policy') ?? '').split(',')
 
   // Note: As the referrer-policy can contain multiple policies
   // separated by comma, we need to loop through all of them
@@ -212,7 +212,7 @@ function appendFetchMetadata (httpRequest) {
   header = httpRequest.mode
 
   //  4. Set a structured field value `Sec-Fetch-Mode`/header in r’s header list.
-  httpRequest.headersList.set('sec-fetch-mode', header)
+  httpRequest.headersList.lowerCaseSet('sec-fetch-mode', header)
 
   //  https://w3c.github.io/webappsec-fetch-metadata/#sec-fetch-site-header
   //  TODO
@@ -229,7 +229,7 @@ function appendRequestOriginHeader (request) {
   // 2. If request’s response tainting is "cors" or request’s mode is "websocket", then append (`Origin`, serializedOrigin) to request’s header list.
   if (request.responseTainting === 'cors' || request.mode === 'websocket') {
     if (serializedOrigin) {
-      request.headersList.append('origin', serializedOrigin)
+      request.headersList.lowerCaseAppend('origin', serializedOrigin)
     }
 
   // 3. Otherwise, if request’s method is neither `GET` nor `HEAD`, then:
@@ -260,7 +260,7 @@ function appendRequestOriginHeader (request) {
 
     if (serializedOrigin) {
       // 2. Append (`Origin`, serializedOrigin) to request’s header list.
-      request.headersList.append('origin', serializedOrigin)
+      request.headersList.lowerCaseAppend('origin', serializedOrigin)
     }
   }
 }