diff --git a/clang-tools-extra/clangd/Preamble.h b/clang-tools-extra/clangd/Preamble.h
--- a/clang-tools-extra/clangd/Preamble.h
+++ b/clang-tools-extra/clangd/Preamble.h
@@ -164,7 +164,7 @@
   static constexpr llvm::StringLiteral HeaderName = "__preamble_patch__.h";
 
   llvm::ArrayRef<PragmaMark> marks() const;
-  MainFileMacros mainFileMacros() const;
+  const MainFileMacros &mainFileMacros() const;
 
 private:
   static PreamblePatch create(llvm::StringRef FileName,
@@ -183,6 +183,7 @@
   PreambleBounds ModifiedBounds = {0, false};
   const PreambleData *Baseline = nullptr;
   std::vector<PragmaMark> PatchedMarks;
+  MainFileMacros PatchedMacros;
 };
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -325,6 +325,7 @@
   std::vector<llvm::StringRef> Lines;
   PreambleBounds Bounds = {0, false};
   std::vector<PragmaMark> Marks;
+  MainFileMacros Macros;
 };
 
 /// Scans the preprocessor directives in the preamble section of the file by
@@ -373,14 +374,15 @@
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0]))
     return error("failed BeginSourceFile");
   Preprocessor &PP = Clang->getPreprocessor();
+  const auto &SM = PP.getSourceManager();
   IncludeStructure Includes;
   Includes.collect(*Clang);
   ScannedPreamble SP;
   SP.Bounds = Bounds;
   PP.addPPCallbacks(
       std::make_unique<DirectiveCollector>(PP, SP.TextualDirectives));
-  PP.addPPCallbacks(
-      collectPragmaMarksCallback(PP.getSourceManager(), SP.Marks));
+  PP.addPPCallbacks(collectPragmaMarksCallback(SM, SP.Marks));
+  PP.addPPCallbacks(std::make_unique<CollectMainFileMacros>(SM, SP.Macros));
   if (llvm::Error Err = Action.Execute())
     return std::move(Err);
   Action.EndSourceFile();
@@ -859,6 +861,7 @@
 
   PP.PatchedDiags = patchDiags(Baseline.Diags, *BaselineScan, *ModifiedScan);
   PP.PatchedMarks = std::move(ModifiedScan->Marks);
+  PP.PatchedMacros = std::move(ModifiedScan->Macros);
   dlog("Created preamble patch: {0}", Patch.str());
   Patch.flush();
   return PP;
@@ -915,11 +918,10 @@
   return PatchedMarks;
 }
 
-MainFileMacros PreamblePatch::mainFileMacros() const {
+const MainFileMacros &PreamblePatch::mainFileMacros() const {
   if (PatchContents.empty())
     return Baseline->Macros;
-  // FIXME: Patch main file macros.
-  return MainFileMacros();
+  return PatchedMacros;
 }
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/PreambleTests.cpp b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
--- a/clang-tools-extra/clangd/unittests/PreambleTests.cpp
+++ b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
@@ -30,7 +30,6 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/VirtualFileSystem.h"
-#include "llvm/Testing/Support/SupportHelpers.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest-matchers.h"
 #include "gtest/gtest.h"
@@ -855,11 +854,12 @@
 ]]
 #pragma $y[[mark YY
 ]]
+#define BAZ
 
 #endif)cpp");
     auto AST = createPatchedAST(Code.code(), NewCode.code());
-    // FIXME: Macros and marks have locations that need to be patched.
-    EXPECT_THAT(AST->getMacros().Names, IsEmpty());
+    EXPECT_THAT(AST->getMacros().Names.keys(),
+                UnorderedElementsAreArray({"FOO", "BAR", "BAZ"}));
     EXPECT_THAT(AST->getMarks(),
                 UnorderedElementsAre(Mark(NewCode.range("x"), " XX"),
                                      Mark(NewCode.range("y"), " YY")));