diff --git a/Help/envvar/CTEST_PARALLEL_LEVEL.rst b/Help/envvar/CTEST_PARALLEL_LEVEL.rst
index 798c79b7e0..ba8ae4f3b2 100644
--- a/Help/envvar/CTEST_PARALLEL_LEVEL.rst
+++ b/Help/envvar/CTEST_PARALLEL_LEVEL.rst
@@ -14,7 +14,6 @@ up to 8 tests concurrently as if ``ctest`` were invoked with the
   parallelism, or unbounded parallelism, respectively, as documented by
   the :option:`ctest --parallel` option.
 
-  On Windows, environment variables cannot be set to an empty string.
   CTest will interpret a whitespace-only string as empty.
 
   In CMake 3.28 and earlier, an empty or ``0`` value was equivalent to ``1``.
diff --git a/Tests/RunCMake/ctest_test/RunCMakeTest.cmake b/Tests/RunCMake/ctest_test/RunCMakeTest.cmake
index dd5c005d18..730efa80b2 100644
--- a/Tests/RunCMake/ctest_test/RunCMakeTest.cmake
+++ b/Tests/RunCMake/ctest_test/RunCMakeTest.cmake
@@ -63,6 +63,7 @@ run_ctest_test(Parallel4     INCLUDE test PARALLEL_LEVEL 4)
 set(ENV{CTEST_PARALLEL_LEVEL} bad)
 run_ctest_test(ParallelEnvBad INCLUDE test)
 if(CMAKE_HOST_WIN32)
+  # FIXME(#27285): `set(ENV{VAR} "")` unsets VAR on Windows.
   set(ENV{CTEST_PARALLEL_LEVEL} " ")
 else()
   set(ENV{CTEST_PARALLEL_LEVEL} "")