diff --git a/gcloud/storage/key.py b/gcloud/storage/key.py
index 0384b368295b..3ddc705fb6a5 100644
--- a/gcloud/storage/key.py
+++ b/gcloud/storage/key.py
@@ -134,10 +134,17 @@ def exists(self):
         return self.bucket.get_key(self.name) is not None
 
     def rename(self, new_name):
-        """Renames this key.
+        """Renames this key using copy and delete operations.
 
         Effectively, copies key to the same bucket with a new name, then
         deletes the key.
+        
+        .. warning::
+          This method will first duplicate the data
+          and then delete the old key.
+          This means that with very large objects
+          renaming could be a very (temporarily) costly
+          or a very slow operation.
 
         :type new_name: string
         :param new_name: The new name for this key.