--- a/mercurial/cmdutil.py	Mon Nov 30 19:52:03 2009 +0100
+++ b/mercurial/cmdutil.py	Tue Nov 17 18:50:39 2009 +0900
@@ -588,7 +588,7 @@
         initfn()
 
     if opts['pid_file']:
-        fp = open(opts['pid_file'], 'w')
+        fp = open(opts['pid_file'], 'a')
         fp.write(str(os.getpid()) + '\n')
         fp.close()