diff --git a/radicale/__main__.py b/radicale/__main__.py
index b2a0da6..175f613 100644
--- a/radicale/__main__.py
+++ b/radicale/__main__.py
@@ -91,6 +91,8 @@ def run():
 
     # Fork if Radicale is launched as daemon
     if options.daemon:
+        if options.pid and os.path.exists(options.pid):
+            raise OSError("PID file exists: %s" % options.pid)
         pid = os.fork()
         if pid:
             try: