diff --git a/clang-tools-extra/clangd/Quality.cpp b/clang-tools-extra/clangd/Quality.cpp
index 00490274ad35..e42fca4e3449 100644
--- a/clang-tools-extra/clangd/Quality.cpp
+++ b/clang-tools-extra/clangd/Quality.cpp
@@ -9,6 +9,7 @@
 #include "Quality.h"
 #include "index/Index.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/Basic/CharInfo.h"
 #include "clang/AST/DeclVisitor.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
@@ -19,6 +20,11 @@
 namespace clang {
 namespace clangd {
 using namespace llvm;
+static bool IsReserved(StringRef Name) {
+  // FIXME: Should we exclude _Bool and others recognized by the standard?
+  return Name.size() >= 2 && Name[0] == '_' &&
+         (isUppercase(Name[1]) || Name[1] == '_');
+}
 
 static bool hasDeclInMainFile(const Decl &D) {
   auto &SourceMgr = D.getASTContext().getSourceManager();
@@ -101,11 +107,18 @@ void SymbolQualitySignals::merge(const CodeCompletionResult &SemaCCResult) {
     Category = categorize(*SemaCCResult.Declaration);
   else if (SemaCCResult.Kind == CodeCompletionResult::RK_Macro)
     Category = Macro;
+
+  if (SemaCCResult.Declaration) {
+    if (auto *ID = SemaCCResult.Declaration->getIdentifier())
+      ReservedName = ReservedName || IsReserved(ID->getName());
+  } else if (SemaCCResult.Kind == CodeCompletionResult::RK_Macro)
+    ReservedName = ReservedName || IsReserved(SemaCCResult.Macro->getName());
 }
 
 void SymbolQualitySignals::merge(const Symbol &IndexResult) {
   References = std::max(IndexResult.References, References);
   Category = categorize(IndexResult.SymInfo);
+  ReservedName = ReservedName || IsReserved(IndexResult.Name);
 }
 
 float SymbolQualitySignals::evaluate() const {
@@ -118,6 +131,8 @@ float SymbolQualitySignals::evaluate() const {
 
   if (Deprecated)
     Score *= 0.1f;
+  if (ReservedName)
+    Score *= 0.1f;
 
   switch (Category) {
     case Type:
@@ -142,6 +157,7 @@ raw_ostream &operator<<(raw_ostream &OS, const SymbolQualitySignals &S) {
   OS << formatv("=== Symbol quality: {0}\n", S.evaluate());
   OS << formatv("\tReferences: {0}\n", S.References);
   OS << formatv("\tDeprecated: {0}\n", S.Deprecated);
+  OS << formatv("\tReserved name: {0}\n", S.ReservedName);
   OS << formatv("\tCategory: {0}\n", static_cast<int>(S.Category));
   return OS;
 }
diff --git a/clang-tools-extra/clangd/Quality.h b/clang-tools-extra/clangd/Quality.h
index c60686286751..ca1f1bf9126e 100644
--- a/clang-tools-extra/clangd/Quality.h
+++ b/clang-tools-extra/clangd/Quality.h
@@ -45,6 +45,8 @@ struct Symbol;
 /// Attributes of a symbol that affect how much we like it.
 struct SymbolQualitySignals {
   bool Deprecated = false;
+  bool ReservedName = false; // __foo, _Foo are usually implementation details.
+                             // FIXME: make these findable once user types _.
   unsigned References = 0;
 
   enum SymbolCategory {
diff --git a/clang-tools-extra/unittests/clangd/QualityTests.cpp b/clang-tools-extra/unittests/clangd/QualityTests.cpp
index 4b16dd59c011..097fd3231556 100644
--- a/clang-tools-extra/unittests/clangd/QualityTests.cpp
+++ b/clang-tools-extra/unittests/clangd/QualityTests.cpp
@@ -28,31 +28,34 @@ namespace {
 
 TEST(QualityTests, SymbolQualitySignalExtraction) {
   auto Header = TestTU::withHeaderCode(R"cpp(
-    int x;
+    int _X;
 
     [[deprecated]]
-    int f() { return x; }
+    int _f() { return _X; }
   )cpp");
   auto Symbols = Header.headerSymbols();
   auto AST = Header.build();
 
   SymbolQualitySignals Quality;
-  Quality.merge(findSymbol(Symbols, "x"));
+  Quality.merge(findSymbol(Symbols, "_X"));
   EXPECT_FALSE(Quality.Deprecated);
+  EXPECT_TRUE(Quality.ReservedName);
   EXPECT_EQ(Quality.References, SymbolQualitySignals().References);
   EXPECT_EQ(Quality.Category, SymbolQualitySignals::Variable);
 
-  Symbol F = findSymbol(Symbols, "f");
+  Symbol F = findSymbol(Symbols, "_f");
   F.References = 24; // TestTU doesn't count references, so fake it.
   Quality = {};
   Quality.merge(F);
   EXPECT_FALSE(Quality.Deprecated); // FIXME: Include deprecated bit in index.
+  EXPECT_FALSE(Quality.ReservedName);
   EXPECT_EQ(Quality.References, 24u);
   EXPECT_EQ(Quality.Category, SymbolQualitySignals::Function);
 
   Quality = {};
-  Quality.merge(CodeCompletionResult(&findDecl(AST, "f"), /*Priority=*/42));
+  Quality.merge(CodeCompletionResult(&findDecl(AST, "_f"), /*Priority=*/42));
   EXPECT_TRUE(Quality.Deprecated);
+  EXPECT_FALSE(Quality.ReservedName);
   EXPECT_EQ(Quality.References, SymbolQualitySignals().References);
   EXPECT_EQ(Quality.Category, SymbolQualitySignals::Function);
 }
@@ -112,6 +115,10 @@ TEST(QualityTests, SymbolQualitySignalsSanity) {
   Deprecated.Deprecated = true;
   EXPECT_LT(Deprecated.evaluate(), Default.evaluate());
 
+  SymbolQualitySignals ReservedName;
+  ReservedName.ReservedName = true;
+  EXPECT_LT(ReservedName.evaluate(), Default.evaluate());
+
   SymbolQualitySignals WithReferences, ManyReferences;
   WithReferences.References = 10;
   ManyReferences.References = 1000;