diff --git a/src/pip/_internal/wheel.py b/src/pip/_internal/wheel.py
index 4a28de4d315b5ca8108ac9ae327b6b6d53b63bf8..170e2bdc517612d5e0a13d6788315e05e4653a9e 100644
--- a/src/pip/_internal/wheel.py
+++ b/src/pip/_internal/wheel.py
@@ -163,13 +163,15 @@ def message_about_scripts_not_on_PATH(scripts):
         grouped_by_dir[parent_dir].add(script_name)
 
     # We don't want to warn for directories that are on PATH.
-    not_warn_dirs = os.environ["PATH"].split(os.pathsep)
+    not_warn_dirs = [
+        os.path.normcase(i) for i in os.environ["PATH"].split(os.pathsep)
+    ]
     # If an executable sits with sys.executable, we don't warn for it.
     #     This covers the case of venv invocations without activating the venv.
-    not_warn_dirs.append(os.path.dirname(sys.executable))
+    not_warn_dirs.append(os.path.normcase(os.path.dirname(sys.executable)))
     warn_for = {
         parent_dir: scripts for parent_dir, scripts in grouped_by_dir.items()
-        if parent_dir not in not_warn_dirs
+        if os.path.normcase(parent_dir) not in not_warn_dirs
     }
     if not warn_for:
         return None
diff --git a/tests/unit/test_wheel.py b/tests/unit/test_wheel.py
index f1ee8e332c19fe20b50e6afb9576ff50c945659e..9b029eedbee47b88056e6ddd54280d0ddd9a04cb 100644
--- a/tests/unit/test_wheel.py
+++ b/tests/unit/test_wheel.py
@@ -453,3 +453,13 @@ class TestMessageAboutScriptsNotOnPATH(object):
             scripts=['/a/b/foo']
         )
         assert retval is None
+
+    def test_PATH_check_case_insensitive_on_windows(self):
+        retval = self._template(
+            paths=['C:\\A\\b'],
+            scripts=['c:\\a\\b\\c', 'C:/A/b/d']
+        )
+        if WINDOWS:
+            assert retval is None
+        else:
+            assert retval is not None