diff --git a/DQMServices/Core/interface/MonitorElement.h b/DQMServices/Core/interface/MonitorElement.h
index 43824e461f38f..6422459c3c73f 100755
--- a/DQMServices/Core/interface/MonitorElement.h
+++ b/DQMServices/Core/interface/MonitorElement.h
@@ -291,6 +291,7 @@ class MonitorElement
     { return data_.flags & DQMNet::DQM_PROP_MARKTODELETE; }
 
   /// Mark the object for deletion.
+  /// NB: make sure that the following method is not called simultaneously for the same ME
   void markToDelete(void)
     { data_.flags |= DQMNet::DQM_PROP_MARKTODELETE; }