diff --git a/collects/scribblings/inside/threads.scrbl b/collects/scribblings/inside/threads.scrbl
index 08d506a06c..ce9cd4368a 100644
--- a/collects/scribblings/inside/threads.scrbl
+++ b/collects/scribblings/inside/threads.scrbl
@@ -553,8 +553,11 @@ it wakes up if the Racket thread if it is sleeping.}
 @function[(void scheme_check_threads)]{
 
 This function is periodically called by the embedding program to give
-background processes time to execute.  See @secref["threadtime"]
-for more information.}
+background processes time to execute. See @secref["threadtime"]
+for more information.
+
+As long as some threads are ready, this functions returns only after
+one thread quantum, at least.}
 
 @function[(void scheme_wake_up)]{
 
diff --git a/src/racket/src/thread.c b/src/racket/src/thread.c
index e31cc8705a..9bc64459c9 100644
--- a/src/racket/src/thread.c
+++ b/src/racket/src/thread.c
@@ -4074,13 +4074,23 @@ void scheme_cancel_sleep()
 }
 
 void scheme_check_threads(void)
-/* Signals should be suspended. */
 {
-  scheme_current_thread->suspend_break++;
-  scheme_thread_block((float)0);
-  --scheme_current_thread->suspend_break;
+  double start, now;
 
-  check_sleep(have_activity, 0);
+  start = scheme_get_inexact_milliseconds();
+  
+  while (1) {
+    scheme_current_thread->suspend_break++;
+    scheme_thread_block((float)0);
+    --scheme_current_thread->suspend_break;
+    
+    if (check_sleep(have_activity, 0))
+      break;
+
+    now = scheme_get_inexact_milliseconds();
+    if (((now - start) * 1000) > MZ_THREAD_QUANTUM_USEC)
+      break;
+  }
 }
 
 void scheme_wake_up(void)