diff --git a/offlineimap/head/debian/changelog b/offlineimap/head/debian/changelog
index 416cd4f..f331f39 100644
--- a/offlineimap/head/debian/changelog
+++ b/offlineimap/head/debian/changelog
@@ -1,3 +1,10 @@
+offlineimap (3.99.9) unstable; urgency=low
+
+  * Added check to make sure that two processes do not run in the same
+    directory at once.
+
+ -- John Goerzen <jgoerzen@complete.org>  Wed, 29 Jan 2003 13:19:14 -0600
+
 offlineimap (3.99.8) unstable; urgency=low
 
   * This is a 4.0 TRACK release, and may be unstable or in flux!
diff --git a/offlineimap/head/offlineimap/init.py b/offlineimap/head/offlineimap/init.py
index 2ba9bbc..4faffd9 100644
--- a/offlineimap/head/offlineimap/init.py
+++ b/offlineimap/head/offlineimap/init.py
@@ -20,11 +20,22 @@ from offlineimap import imaplib, imapserver, repository, folder, mbnames, thread
 from offlineimap.localeval import LocalEval
 from offlineimap.threadutil import InstanceLimitedThread, ExitNotifyThread
 from offlineimap.ui import UIBase
-import re, os, os.path, offlineimap, sys
+import re, os, os.path, offlineimap, sys, fcntl
 from offlineimap.CustomConfig import CustomConfigParser
 from threading import *
 from getopt import getopt
 
+lockfd = None
+
+def lock(config, ui):
+    global lockfd
+    lockfd = open(config.getmetadatadir() + "/lock", "w")
+    try:
+        fcntl.flock(lockfd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except IOError:
+        ui.locked()
+        ui.terminate(1)
+
 def startup(versionno):
     assert versionno == version.versionstr, "Revision of main program (%d) does not match that of library (%d).  Please double-check your PYTHONPATH and installation locations." % (revno, version.revno)
     options = {}
@@ -72,6 +83,8 @@ def startup(versionno):
         for section in config.getaccountlist():
             config.remove_option(section, "autorefresh")
 
+    lock(config, ui)
+
     accounts = config.get("general", "accounts")
     if '-a' in options:
         accounts = options['-a']
diff --git a/offlineimap/head/offlineimap/ui/UIBase.py b/offlineimap/head/offlineimap/ui/UIBase.py
index 776275a..0eae1a2 100644
--- a/offlineimap/head/offlineimap/ui/UIBase.py
+++ b/offlineimap/head/offlineimap/ui/UIBase.py
@@ -101,6 +101,9 @@ class UIBase:
     def invaliddebug(s, debugtype):
         s.warn("Invalid debug type: %s" % debugtype)
 
+    def locked(s):
+        s.warn("Another OfflineIMAP is running with the same metadatadir; exiting.")
+
     def getnicename(s, object):
         prelimname = str(object.__class__).split('.')[-1]
         # Strip off extra stuff.