diff --git a/include/grpc++/test/mock_stream.h b/include/grpc++/test/mock_stream.h
index e1dbd5ae3f475b127cc808e09a8eaabd1ae41f7f..c26ddecb705eae5497f46d928f834a87e7befe76 100644
--- a/include/grpc++/test/mock_stream.h
+++ b/include/grpc++/test/mock_stream.h
@@ -63,6 +63,8 @@ class MockClientReaderWriter : public ClientReaderWriterInterface<W, R> {
   MOCK_METHOD0_T(WritesDone, bool());
 };
 
+// TODO: We do not support mocking an async RPC for now.
+
 template <class R>
 class MockClientAsyncResponseReader
     : public ClientAsyncResponseReaderInterface<R> {