diff --git a/winsup/doc/how-using.texinfo b/winsup/doc/how-using.texinfo
index 963480001..d5df8e6f6 100644
--- a/winsup/doc/how-using.texinfo
+++ b/winsup/doc/how-using.texinfo
@@ -110,6 +110,16 @@ itself a symbolic link, then find will not traverse it unless you
 specify the @samp{-follow} option.  This behavior is different than most
 other UNIX implementations, but is not likely to change.
 
+@subsection Where is the @samp{su} command?
+
+The @samp{su} command is not ported to Cygwin, so it is no longer
+provided in the distribution.  You may be able to use @samp{login}
+instead, but you should read
+@file{http://www.cygwin.com/ml/cygwin/2001-03/msg00337.html} first.
+
+If you have a copy of su, then it's from an old Cygwin distribution, and
+it probably doesn't work properly anyway.
+
 @subsection Why doesn't man (or apropos) work?
 
 Even after installing the @samp{man} package, you get an error like this: