diff --git a/clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp b/clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp
--- a/clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp
+++ b/clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp
@@ -175,6 +175,12 @@
 
 bool AddUsing::prepare(const Selection &Inputs) {
   auto &SM = Inputs.AST->getSourceManager();
+
+  // Do not suggest "using" in header files. That way madness lies.
+  if (isHeaderFile(SM.getFileEntryForID(SM.getMainFileID())->getName(),
+                   Inputs.AST->getLangOpts()))
+    return false;
+
   auto *Node = Inputs.ASTSelection.commonAncestor();
   if (Node == nullptr)
     return false;
diff --git a/clang-tools-extra/clangd/unittests/TweakTests.cpp b/clang-tools-extra/clangd/unittests/TweakTests.cpp
--- a/clang-tools-extra/clangd/unittests/TweakTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TweakTests.cpp
@@ -2463,6 +2463,13 @@
   // test that we don't crash.
   EXPECT_UNAVAILABLE(Header +
                      "template<typename TT> using foo = one::tt<T^T>;");
+
+  // Check that we do not trigger in header files.
+  FileName = "test.h";
+  ExtraArgs.push_back("-xc++-header"); // .h file is treated a C by default.
+  EXPECT_UNAVAILABLE(Header + "void fun() { one::two::f^f(); }");
+  FileName = "test.hpp";
+  EXPECT_UNAVAILABLE(Header + "void fun() { one::two::f^f(); }");
 }
 
 TEST_F(AddUsingTest, Apply) {