diff --git a/docs/CMake.rst b/docs/CMake.rst
index b8a5b3112ed..7ec67603c07 100644
--- a/docs/CMake.rst
+++ b/docs/CMake.rst
@@ -186,6 +186,8 @@ CMake manual, or execute ``cmake --help-variable VARIABLE_NAME``.
   Sets the build type for ``make``-based generators. Possible values are
   Release, Debug, RelWithDebInfo and MinSizeRel. If you are using an IDE such as
   Visual Studio, you should use the IDE settings to set the build type.
+  Be aware that Release and RelWithDebInfo are not using the same optimization
+  level on most platform.
 
 **CMAKE_INSTALL_PREFIX**:PATH
   Path where LLVM will be installed if "make install" is invoked or the