diff --git a/collects/mred/private/messagebox.rkt b/collects/mred/private/messagebox.rkt
index b598fbccd2..38733d0ffb 100644
--- a/collects/mred/private/messagebox.rkt
+++ b/collects/mred/private/messagebox.rkt
@@ -41,6 +41,27 @@
 			 l))
 		   style)
 
+      (let ([go (lambda ()
+                  (create-message-box/custom
+                   who title message
+                   button1 button2 button3
+                   parent style close-result
+                   check? two-results? check-message))])
+        (if (eq? (current-thread) (wx:eventspace-handler-thread (wx:current-eventspace)))
+            ;; In the right thread:
+            (go)
+            ;; Not in the right thread:
+            (let ([ch (make-channel)])
+              (wx:queue-callback
+               (lambda ()
+                 (channel-put ch (call-with-values go list))))
+              (apply values (channel-get ch)))))))
+
+  (define create-message-box/custom
+    (lambda (who title message
+		 button1 button2 button3
+		 parent style close-result
+		 check? two-results? check-message)
       (let* ([strings (regexp-split #rx"\n" message)]
 	     [single? (and (< (length strings) 10) 
 			   (andmap (lambda (s) (< (string-length s) 60)) strings))]
@@ -212,7 +233,7 @@
 			     #f #f #f)))
 
   (define do-message-box
-    (opt-lambda (who title message parent style check? check-message)
+    (lambda (who title message parent style check? check-message)
       (check-label-string who title)
       (check-string/false who message)
       (when check?
diff --git a/collects/scribblings/gui/dialog-funcs.scrbl b/collects/scribblings/gui/dialog-funcs.scrbl
index 1644637a3d..47b2e262f4 100644
--- a/collects/scribblings/gui/dialog-funcs.scrbl
+++ b/collects/scribblings/gui/dialog-funcs.scrbl
@@ -180,10 +180,9 @@ See also @scheme[path-dialog%].
                       [parent (or/c (is-a?/c frame%) (is-a?/c dialog%) false/c) #f]
                       [style (listof (one-of/c 'ok 'ok-cancel 'yes-no 'caution 'stop)) '(ok)])
          (one-of/c 'ok 'cancel 'yes 'no)]{
+
 See also @scheme[message-box/custom].
 
-
-
 Displays a message to the user in a (modal) dialog, using
  @scheme[parent] as the parent window if it is specified. The dialog's
  title is @scheme[title]. The @scheme[message] string can be arbitrarily
@@ -220,12 +219,11 @@ In addition, @scheme[style] can contain @scheme['caution] to make the
 The class that implements the dialog provides a @scheme[get-message]
  method that takes no arguments and returns the text of the message as
  a string. (The dialog is accessible through the
-@scheme[get-top-level-windows] function.)
+ @scheme[get-top-level-windows] function.)
 
-
-
-
-}
+The @racket[message-box] function can be called int a thread other
+ than the current eventspace's handler thread, in which case the
+ current thread blocks while the dialog runs on the handler thread.}
 
 @defproc[(message-box/custom [title label-string?]
                              [message string]
@@ -307,9 +305,9 @@ The class that implements the dialog provides a @scheme[get-message]
  a string. (The dialog is accessible through the
 @scheme[get-top-level-windows] function.)
 
-
-
-}
+The @racket[message-box/custom] function can be called int a thread
+ other than the current eventspace's handler thread, in which case the
+ current thread blocks while the dialog runs on the handler thread.}
 
 @defproc[(message+check-box [title label-string?]
                             [message string]