diff --git a/tonic-health/proto/health.proto b/tonic-health/proto/health.proto
index 871b3d7..a8c10ab 100644
--- a/tonic-health/proto/health.proto
+++ b/tonic-health/proto/health.proto
@@ -11,6 +11,7 @@ message HealthCheckResponse {
         UNKNOWN = 0;
         SERVING = 1;
         NOT_SERVING = 2;
+        SERVICE_UNKNOWN = 3;  // Used only by the Watch method.
     }
     ServingStatus status = 1;
 }
@@ -19,4 +20,4 @@ service Health {
     rpc Check(HealthCheckRequest) returns (HealthCheckResponse);
 
     rpc Watch(HealthCheckRequest) returns (stream HealthCheckResponse);
-}
\ No newline at end of file
+}