diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 1af14cc..2d8aa50 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -349,6 +349,9 @@
       ...
       cfgparser.optionxform = str
 
+   Note that when reading configuration files, whitespace around the
+   option names are stripped before :meth:``optionxform`` is called.
+
 
 .. _configparser-objects: