diff --git a/src/lwt/trace_lwt.ml b/src/lwt/trace_lwt.ml
index 3962e0a..31c9487 100644
--- a/src/lwt/trace_lwt.ml
+++ b/src/lwt/trace_lwt.ml
@@ -45,10 +45,10 @@ let[@inline never] with_span_real_ ?parent ?(force_toplevel = false)
   let espan =
     match parent, force_toplevel with
     | _, true | None, _ ->
-      enter_manual_toplevel_span ~flavor:`Sync ?__FUNCTION__ ~__FILE__ ~__LINE__
-        ?data name
+      enter_manual_toplevel_span ~flavor:`Async ?__FUNCTION__ ~__FILE__
+        ~__LINE__ ?data name
     | Some parent, _ ->
-      enter_manual_sub_span ~parent ~flavor:`Sync ?__FUNCTION__ ~__FILE__
+      enter_manual_sub_span ~parent ~flavor:`Async ?__FUNCTION__ ~__FILE__
         ~__LINE__ ?data name
   in
 
diff --git a/src/lwt/trace_lwt.mli b/src/lwt/trace_lwt.mli
index f41a660..76f5398 100644
--- a/src/lwt/trace_lwt.mli
+++ b/src/lwt/trace_lwt.mli
@@ -38,7 +38,8 @@ val with_span :
   'a
 (** [with_span ~__FILE__ ~__LINE__ name f] calls [f span]
     where [span] is a new span named with [name]. The span is
-    traced as being synchronous.
+    traced as being asynchronous, so each collector might represent
+    it differently.
 
     If [f] returns an ['a Lwt.t] future, use [with_span_lwt] instead.