diff --git a/rcl/include/rcl/client.h b/rcl/include/rcl/client.h
index 0f03a506d..b9c70a875 100644
--- a/rcl/include/rcl/client.h
+++ b/rcl/include/rcl/client.h
@@ -257,6 +257,8 @@ rcl_send_request(const rcl_client_t * client, const void * ros_request, int64_t
  * occur.
  * The request_header is an rmw struct for meta-information about the request
  * sent (e.g. the sequence number).
+ * The caller must provide a pointer to an allocated struct.
+ * This function will populate the struct's fields.
  * `ros_response` should point to an already allocated ROS response message
  * struct of the correct type, into which the response from the service will be
  * copied.