diff --git a/runtime/doc/undo.txt b/runtime/doc/undo.txt
index 96ff96a..e8fed4f 100644
--- a/runtime/doc/undo.txt
+++ b/runtime/doc/undo.txt
@@ -225,6 +225,9 @@
 Undo files are normally saved in the same directory as the file.  This can be
 changed with the 'undodir' option.
 
+When the file is encrypted, the text in the undo file is also crypted.  The
+same key and method is used. |encryption|
+
 You can also save and restore undo histories by using ":wundo" and ":rundo"
 respectively:
 							*:wundo* *:rundo*