diff --git a/demo/dask/cpu_survival.py b/demo/dask/cpu_survival.py
index 8bf464ce21d3..44032bab207f 100644
--- a/demo/dask/cpu_survival.py
+++ b/demo/dask/cpu_survival.py
@@ -6,6 +6,7 @@
 
 import os
 
+import dask.array as da
 import dask.dataframe as dd
 from dask.distributed import Client, LocalCluster
 
@@ -13,7 +14,7 @@
 from xgboost.dask import DaskDMatrix
 
 
-def main(client):
+def main(client: Client) -> da.Array:
     # Load an example survival data from CSV into a Dask data frame.
     # The Veterans' Administration Lung Cancer Trial
     # The Statistical Analysis of Failure Time Data by Kalbfleisch J. and Prentice R (1980)
diff --git a/demo/dask/cpu_training.py b/demo/dask/cpu_training.py
index 7117eddd9b51..59471da7c6ed 100644
--- a/demo/dask/cpu_training.py
+++ b/demo/dask/cpu_training.py
@@ -11,7 +11,7 @@
 from xgboost.dask import DaskDMatrix
 
 
-def main(client):
+def main(client: Client) -> None:
     # generate some random data for demonstration
     m = 100000
     n = 100
diff --git a/demo/dask/dask_callbacks.py b/demo/dask/dask_callbacks.py
index 4a7ec0f191cb..1a15b918a534 100644
--- a/demo/dask/dask_callbacks.py
+++ b/demo/dask/dask_callbacks.py
@@ -3,6 +3,8 @@
 ====================================
 """
 
+from typing import Any
+
 import numpy as np
 from dask.distributed import Client, LocalCluster
 from dask_ml.datasets import make_regression
@@ -13,7 +15,7 @@
 from xgboost.dask import DaskDMatrix
 
 
-def probability_for_going_backward(epoch):
+def probability_for_going_backward(epoch: int) -> float:
     return 0.999 / (1.0 + 0.05 * np.log(1.0 + epoch))
 
 
@@ -23,7 +25,9 @@ class CustomEarlyStopping(xgb.callback.TrainingCallback):
     In the beginning, allow the metric to become worse with a probability of 0.999.
     As boosting progresses, the probability should be adjusted downward"""
 
-    def __init__(self, *, validation_set, target_metric, maximize, seed):
+    def __init__(
+        self, *, validation_set: str, target_metric: str, maximize: bool, seed: int
+    ) -> None:
         self.validation_set = validation_set
         self.target_metric = target_metric
         self.maximize = maximize
@@ -34,7 +38,9 @@ def __init__(self, *, validation_set, target_metric, maximize, seed):
         else:
             self.better = lambda x, y: x < y
 
-    def after_iteration(self, model, epoch, evals_log):
+    def after_iteration(
+        self, model: Any, epoch: int, evals_log: xgb.callback.TrainingCallback.EvalsLog
+    ) -> bool:
         metric_history = evals_log[self.validation_set][self.target_metric]
         if len(metric_history) < 2 or self.better(
             metric_history[-1], metric_history[-2]
@@ -42,7 +48,7 @@ def after_iteration(self, model, epoch, evals_log):
             return False  # continue training
         p = probability_for_going_backward(epoch)
         go_backward = self.rng.choice(2, size=(1,), replace=True, p=[1 - p, p]).astype(
-            np.bool
+            np.bool_
         )[0]
         print(
             "The validation metric went into the wrong direction. "
@@ -54,7 +60,7 @@ def after_iteration(self, model, epoch, evals_log):
             return True  # stop training
 
 
-def main(client):
+def main(client: Client) -> None:
     m = 100000
     n = 100
     X, y = make_regression(n_samples=m, n_features=n, chunks=200, random_state=0)
diff --git a/demo/dask/sklearn_cpu_training.py b/demo/dask/sklearn_cpu_training.py
index e91babb8407b..38a53c6ca71c 100644
--- a/demo/dask/sklearn_cpu_training.py
+++ b/demo/dask/sklearn_cpu_training.py
@@ -9,7 +9,7 @@
 from xgboost import dask as dxgb
 
 
-def main(client):
+def main(client: Client) -> dxgb.Booster:
     # generate some random data for demonstration
     n = 100
     m = 10000
diff --git a/demo/dask/sklearn_gpu_training.py b/demo/dask/sklearn_gpu_training.py
index 7686909951e6..6161bf9a3402 100644
--- a/demo/dask/sklearn_gpu_training.py
+++ b/demo/dask/sklearn_gpu_training.py
@@ -12,7 +12,7 @@
 from xgboost import dask as dxgb
 
 
-def main(client):
+def main(client: Client) -> dxgb.Booster:
     # generate some random data for demonstration
     n = 100
     m = 1000000
diff --git a/tests/ci_build/lint_python.py b/tests/ci_build/lint_python.py
index 741ef7558f13..d56191dc4566 100644
--- a/tests/ci_build/lint_python.py
+++ b/tests/ci_build/lint_python.py
@@ -98,6 +98,7 @@ class LintersPaths:
         "tests/test_distributed/test_gpu_with_spark/test_data.py",
         "tests/test_distributed/test_gpu_with_dask/test_gpu_with_dask.py",
         # demo
+        "demo/dask/",
         "demo/json-model/json_parser.py",
         "demo/guide-python/external_memory.py",
         "demo/guide-python/sklearn_examples.py",