diff --git a/doc/health-checking.md b/doc/health-checking.md
index 22b6e1b4c09..0d866e3f4db 100644
--- a/doc/health-checking.md
+++ b/doc/health-checking.md
@@ -37,6 +37,7 @@ message HealthCheckResponse {
     UNKNOWN = 0;
     SERVING = 1;
     NOT_SERVING = 2;
+    SERVICE_UNKNOWN = 3;  // Used only by the Watch method.
   }
   ServingStatus status = 1;
 }