diff --git a/src/testing/testing.go b/src/testing/testing.go
index 5b0a33f28d..b03e6fdeda 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -2043,6 +2043,9 @@ var testingTesting bool
 var realStderr *os.File
 
 // Run runs the tests. It returns an exit code to pass to os.Exit.
+// The exit code is zero when all tests pass, and non-zero for any kind
+// of failure. For machine readable test results, parse the output of
+// 'go test -json'.
 func (m *M) Run() (code int) {
 	defer func() {
 		code = m.exitCode