Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -1469,7 +1469,8 @@
                       std::inserter(Result, Result.begin()));
 
   llvm::Regex IncludeRegex(IncludeRegexPattern);
-  llvm::Regex DefineRegex(R"(^[\t\ ]*#[\t\ ]*define[\t\ ]*[^\\]*$)");
+  llvm::Regex IfNDefRegex(R"(^[\t\ ]*#[\t\ ]*ifndef[\t\ ]*([^\\]*)$)");
+  llvm::Regex DefineRegex(R"(^[\t\ ]*#[\t\ ]*define[\t\ ]*([^\\]*)$)");
   SmallVector<StringRef, 4> Matches;
 
   StringRef FileName = Replaces.begin()->getFilePath();
@@ -1487,7 +1488,9 @@
   int AfterHeaderGuard = 0;
   SmallVector<StringRef, 32> Lines;
   Code.split(Lines, '\n');
-  for (auto Line : Lines) {
+  for (auto I = Lines.begin(), B = Lines.begin(), E = Lines.end(); I != E;
+       ++I) {
+    StringRef Line = *I;
     if (IncludeRegex.match(Line, &Matches)) {
       StringRef IncludeName = Matches[2];
       int Category = Categories.getIncludePriority(
@@ -1497,9 +1500,13 @@
         FirstIncludeOffset = Offset;
     }
     Offset += Line.size() + 1;
-    // FIXME: make header guard matching stricter, e.g. consider #ifndef.
-    if (AfterHeaderGuard == 0 && DefineRegex.match(Line))
-      AfterHeaderGuard = Offset;
+    if (FirstIncludeOffset < 0 && AfterHeaderGuard == 0 && I != B &&
+        DefineRegex.match(Line, &Matches)) {
+      StringRef DefineName = Matches[1];
+      if (IfNDefRegex.match(*std::prev(I), &Matches) &&
+          DefineName == Matches[1])
+        AfterHeaderGuard = Offset;
+    }
   }
 
   // Populate CategoryEndOfssets:
Index: unittests/Format/CleanupTest.cpp
===================================================================
--- unittests/Format/CleanupTest.cpp
+++ unittests/Format/CleanupTest.cpp
@@ -537,6 +537,34 @@
   EXPECT_EQ(Expected, formatAndApply(Code, Replaces));
 }
 
+TEST_F(CleanUpReplacementsTest, DontJustInsertAfterRandomDefine) {
+  std::string Code = "#define X 1";
+  std::string Expected = "#include <vector>\n"
+                         "#define X 1";
+  tooling::Replacements Replaces = {createInsertion("#include <vector>")};
+  EXPECT_EQ(Expected, apply(Code, Replaces));
+}
+
+TEST_F(CleanUpReplacementsTest, InsertAfterHeaderGuard) {
+  std::string Code = "#ifndef X_H\n"
+                     "#define X_H\n";
+  std::string Expected = "#ifndef X_H\n"
+                         "#define X_H\n"
+                         "#include <vector>\n";
+  tooling::Replacements Replaces = {createInsertion("#include <vector>")};
+  EXPECT_EQ(Expected, apply(Code, Replaces));
+}
+
+TEST_F(CleanUpReplacementsTest, DontInsertAfterNonMatchedHeaderGuard) {
+  std::string Code = "#ifndef X_H\n"
+                     "#define Y_H";
+  std::string Expected = "#include <vector>\n"
+                         "#ifndef X_H\n"
+                         "#define Y_H";
+  tooling::Replacements Replaces = {createInsertion("#include <vector>")};
+  EXPECT_EQ(Expected, apply(Code, Replaces));
+}
+
 } // end namespace
 } // end namespace format
 } // end namespace clang