diff --git a/balancer/balancer.go b/balancer/balancer.go
index a642b15e..ee1703f0 100644
--- a/balancer/balancer.go
+++ b/balancer/balancer.go
@@ -155,7 +155,8 @@ type PickOptions struct {
 	// FullMethodName is the method name that NewClientStream() is called
 	// with. The canonical format is /service/Method.
 	FullMethodName string
-	// Header contains the metadata from the RPC's client header.
+	// Header contains the metadata from the RPC's client header.  The metadata
+	// should not be modified; make a copy first if needed.
 	Header metadata.MD
 }