diff --git a/README.md b/README.md
index 970b79a..03ad084 100644
--- a/README.md
+++ b/README.md
@@ -105,7 +105,7 @@ Bugs, issues and contributions can be requested to both the mailing list or the
 * six (required)
 * imaplib2 >= 2.57 (optional)
 * gssapi (optional), for Kerberos authentication
-
+* portalocker(optional), if you need to run offlineimap in Cygwin for Windows
 
 ## Documentation
 
diff --git a/requirements.txt b/requirements.txt
index 32755b4..f4527a3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
 # Requirements
 six
 gssapi[kerberos]
+portalocker