diff --git a/clang/include/clang/Lex/PPCallbacks.h b/clang/include/clang/Lex/PPCallbacks.h
index d2e3f353cbfd..ddd7415c2936 100644
--- a/clang/include/clang/Lex/PPCallbacks.h
+++ b/clang/include/clang/Lex/PPCallbacks.h
@@ -64,6 +64,11 @@ public:
   /// MacroDefined - This hook is called whenever a macro definition is seen.
   virtual void MacroDefined(const IdentifierInfo *II, const MacroInfo *MI) {
   }
+
+  /// MacroUndefined - This hook is called whenever a macro #undef is seen.
+  /// MI is released immediately following this callback.
+  virtual void MacroUndefined(const IdentifierInfo *II, const MacroInfo *MI) {
+  }
 };
 
 }  // end namespace clang
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 15477b63d3a6..a6e1e7ee2ca0 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1466,7 +1466,11 @@ void Preprocessor::HandleUndefDirective(Token &UndefTok) {
 
   if (!MI->isUsed())
     Diag(MI->getDefinitionLoc(), diag::pp_macro_not_used);
-  
+
+  // If the callbacks want to know, tell them about the macro #undef.
+  if (Callbacks)
+    Callbacks->MacroUndefined(MacroNameTok.getIdentifierInfo(), MI);
+
   // Free macro definition.
   ReleaseMacroInfo(MI);
   setMacroInfo(MacroNameTok.getIdentifierInfo(), 0);