diff --git a/sdk/lib/async/timer.dart b/sdk/lib/async/timer.dart
index 186bbe62abf3..aa2f08864e04 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -89,6 +89,10 @@ abstract class Timer {
 
   /**
    * Cancels the timer.
+   *
+   * Once a [Timer] has been canceled, the callback function will not be called
+   * by the timer. Calling [cancel] more than once on a [Timer] is allowed, and
+   * will have no further effect.
    */
   void cancel();