diff --git a/internal/lsp/diagnostics.go b/internal/lsp/diagnostics.go
index db8eeb59cd..f4ff11d437 100644
--- a/internal/lsp/diagnostics.go
+++ b/internal/lsp/diagnostics.go
@@ -44,6 +44,14 @@ func (s *Server) diagnose(ctx context.Context, snapshot source.Snapshot, alwaysA
 	ctx, done := trace.StartSpan(ctx, "lsp:background-worker")
 	defer done()
 
+	// Wait for a free diagnostics slot.
+	select {
+	case <-ctx.Done():
+		return nil
+	case s.diagnosticsSema <- struct{}{}:
+	}
+	defer func() { <-s.diagnosticsSema }()
+
 	allReports := make(map[diagnosticKey][]source.Diagnostic)
 	var reportsMu sync.Mutex
 	var wg sync.WaitGroup
diff --git a/internal/lsp/lsp_test.go b/internal/lsp/lsp_test.go
index a893834575..d6febccd16 100644
--- a/internal/lsp/lsp_test.go
+++ b/internal/lsp/lsp_test.go
@@ -85,12 +85,9 @@ func testLSP(t *testing.T, exporter packagestest.Exporter) {
 			t.Fatal(err)
 		}
 		r := &runner{
-			server: &Server{
-				session:   session,
-				delivered: map[span.URI]sentDiagnostics{},
-			},
-			data: datum,
-			ctx:  ctx,
+			server: NewServer(session, nil),
+			data:   datum,
+			ctx:    ctx,
 		}
 		t.Run(datum.Folder, func(t *testing.T) {
 			t.Helper()
diff --git a/internal/lsp/server.go b/internal/lsp/server.go
index ed1c00c935..1aad63af5b 100644
--- a/internal/lsp/server.go
+++ b/internal/lsp/server.go
@@ -16,13 +16,16 @@ import (
 	"golang.org/x/tools/internal/span"
 )
 
+const concurrentAnalyses = 1
+
 // NewServer creates an LSP server and binds it to handle incoming client
 // messages on on the supplied stream.
 func NewServer(session source.Session, client protocol.Client) *Server {
 	return &Server{
-		delivered: make(map[span.URI]sentDiagnostics),
-		session:   session,
-		client:    client,
+		delivered:       make(map[span.URI]sentDiagnostics),
+		session:         session,
+		client:          client,
+		diagnosticsSema: make(chan struct{}, concurrentAnalyses),
 	}
 }
 
@@ -54,6 +57,9 @@ type Server struct {
 	// delivered is a cache of the diagnostics that the server has sent.
 	deliveredMu sync.Mutex
 	delivered   map[span.URI]sentDiagnostics
+
+	// diagnosticsSema limits the concurrency of diagnostics runs, which can be expensive.
+	diagnosticsSema chan struct{}
 }
 
 // sentDiagnostics is used to cache diagnostics that have been sent for a given file.