diff --git a/mlir/lib/Tools/mlir-opt/MlirOptMain.cpp b/mlir/lib/Tools/mlir-opt/MlirOptMain.cpp
--- a/mlir/lib/Tools/mlir-opt/MlirOptMain.cpp
+++ b/mlir/lib/Tools/mlir-opt/MlirOptMain.cpp
@@ -337,7 +337,8 @@
   cl::ParseCommandLineOptions(argc, argv, helpHeader);
   MlirOptMainConfig config = MlirOptMainConfig::createFromCLOptions();
   config.preloadDialectsInContext(preloadDialectsInContext);
-
+  if (inputFilename == "-")
+    llvm::errs() << "(processing input from stdin now, hit ctrl-c/ctrl-d to interrupt)\n";
   // Set up the input file.
   std::string errorMessage;
   auto file = openInputFile(inputFilename, &errorMessage);