diff --git a/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/annotate.rkt b/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/annotate.rkt
index 987285218c..f085735a87 100644
--- a/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/annotate.rkt
+++ b/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/annotate.rkt
@@ -25,10 +25,10 @@
   (when defs
     (send defs syncheck:color-range source start finish style-name)))
 
-;; add-mouse-over : syntax[original] string -> void
+;; add-mouse-over : syntax[original] (or/c string (-> string?)) -> void
 ;; registers the range in the editor so that a mouse over
 ;; this area shows up in the status line.
-(define (add-mouse-over stx str)
+(define (add-mouse-over stx str/proc)
   (define source (find-source-editor stx))
   (define defs-text (current-annotations))
   (when (and defs-text 
@@ -37,12 +37,19 @@
              (syntax-span stx))
     (define pos-left (- (syntax-position stx) 1))
     (define pos-right (+ pos-left (syntax-span stx)))
-    (send defs-text syncheck:add-mouse-over-status
-          source pos-left pos-right str)))
+    (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)))
 
-(define (add-mouse-over/loc source pos-left pos-right str)
+(define (add-mouse-over/loc source pos-left pos-right str/proc)
   (define defs-text (current-annotations))
   (when defs-text
+    (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)))
+
+(define (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)
+  (define str
+    (cond
+      [(procedure? str/proc) (str/proc)]
+      [else str/proc]))
+  (when (string? str)
     (send defs-text syncheck:add-mouse-over-status
           source pos-left pos-right str)))
 
diff --git a/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/traversals.rkt b/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/traversals.rkt
index fce049e9e9..d87aec2429 100644
--- a/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/traversals.rkt
+++ b/pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/traversals.rkt
@@ -545,7 +545,10 @@
             prop)])))
 
 (define mouse-over-tooltip-prop?
-  (vector/c #:flat? #t syntax? exact-nonnegative-integer? exact-nonnegative-integer? string?))
+  (vector/c #:flat? #t syntax? exact-nonnegative-integer? exact-nonnegative-integer?
+            (or/c string?
+                  ;; don't check the range here, since we want a predicate, not really a contract
+                  (-> any))))
 (define (add-mouse-over-tooltips stx)
   (let loop ([prop (syntax-property stx 'mouse-over-tooltips)])
     (cond
diff --git a/pkgs/drracket-pkgs/drracket/scribblings/tools/tools.scrbl b/pkgs/drracket-pkgs/drracket/scribblings/tools/tools.scrbl
index a2b67b08c5..de7ba76925 100644
--- a/pkgs/drracket-pkgs/drracket/scribblings/tools/tools.scrbl
+++ b/pkgs/drracket-pkgs/drracket/scribblings/tools/tools.scrbl
@@ -753,12 +753,18 @@ After putting this code in the DrRacket window, mouse over the words ``big'' and
 The value of the @racket['mouse-over-tooltips] property is expected to be 
 to be a tree of @racket[cons] pairs (in any configuration) whose leaves
 are either ignored or are vectors of the shape
-@racketblock[(vector/c syntax? exact-nonnegative-integer? exact-nonnegative-integer? string?)]
+@racketblock[(vector/c syntax?
+                       exact-nonnegative-integer?
+                       exact-nonnegative-integer?
+                       (or/c string? (-> string?)))]
 Each vector's content indicates where to show a tooltip. The first three components are
 a syntax object whose @racket[syntax-source] field indicates which file the tooltip goes in,
 the start and end position in the editor where mouseovers will show the tooltip, 
-and the content of the tooltip. For example, here's a macro that shows the span of itself 
-in a tooltip on mouseover:
+and the content of the tooltip. If the tooltip content is a procedure, this procedure
+is called by Check Syntax to compute the string used for the tooltip, as Check Syntax
+traverses the syntax objects looking for properties.
+
+For example, here's a macro that shows the span of itself in a tooltip on mouseover:
 @codeblock{
 #lang racket
 (define-syntax (char-span stx)
@@ -777,7 +783,7 @@ in a tooltip on mouseover:
 
 (char-span (+ 1 2))}
 
-Finally, Check Syntax only draws arrows between identifiers that are @racket[syntax-original?]
+Finally, Check Syntax draws arrows only between identifiers that are @racket[syntax-original?]
 or that have the @racket[syntax-property] @racket['original-for-check-syntax]
 set to @racket[#t].