diff --git a/collects/syntax/doc.txt b/collects/syntax/doc.txt
index f51b9efd86..11fc8ea87a 100644
--- a/collects/syntax/doc.txt
+++ b/collects/syntax/doc.txt
@@ -603,3 +603,10 @@ _to-string.ss_: rendering syntax objects with formatting
   and indenting according to the source locations in stx-list
 
   the outer pair of parens are not rendered from stx-list.
+
+======================================================================
+_free-vars.ss_: computing the free variables of an expression
+======================================================================
+
+> (free-vars expr-stx) - Returns a list of free lambda- and let-bound
+  identifiers in expr-stx. The expression must be fully expanded.
diff --git a/collects/syntax/free-vars.ss b/collects/syntax/free-vars.ss
new file mode 100644
index 0000000000..4c62bf5e58
--- /dev/null
+++ b/collects/syntax/free-vars.ss
@@ -0,0 +1,75 @@
+#lang scheme/base
+;; this finds the free variables of fully-expanded mzscheme expressions
+;; adapted from code by mflatt
+
+
+(require syntax/kerncase
+         syntax/boundmap
+         (for-template scheme/base))
+
+(provide free-vars)
+
+;; merge : (liftof (listof id)) -> (listof id)
+;;  merges lists of identifiers, removing module-identifier=?
+;;  duplicates
+(define (merge l)
+  (cond
+    [(null? l) null]
+    [(null? (cdr l)) (car l)]
+    [else (let ([m (make-module-identifier-mapping)])
+            (for-each (lambda (ids)
+                        (for-each (lambda (id)
+                                    (module-identifier-mapping-put! m id #t))
+                                  ids))
+                      l)
+            (module-identifier-mapping-map m (lambda (k v) k)))]))
+
+;; formals->boundmap : formals-stx -> bound-map-table
+;; Parses a procedure "formals" and returns the binding ids
+;;  in a table
+(define (formals->boundmap f)
+  (let ([ids (let loop ([f f])
+               (cond
+                 [(identifier? f) (list f)]
+                 [(pair? f) (cons (car f)
+                                  (loop (cdr f)))]
+                 [(null? f) null]
+                 [(syntax? f) (loop (syntax-e f))]))]
+        [b (make-bound-identifier-mapping)])
+    (for-each (lambda (id)
+                (bound-identifier-mapping-put! b id #t))
+              ids)
+    b))
+
+;; free-vars : expr-stx -> (listof id)
+;; Returns a list of free lambda- and let-bound identifiers in a
+;;  given epression. The expression must be fully expanded.
+(define (free-vars e)
+  (kernel-syntax-case e #f
+    [id 
+     (identifier? #'id) 
+     (if (eq? 'lexical (identifier-binding #'id))
+         (list #'id)
+         null)]
+    [(#%top . id) null]
+    [(#%expression e) (free-vars #'e)]
+    [(quote q) null]
+    [(quote-syntax q) null]
+    [(#%plain-lambda formals expr ...)
+     (let ([free (merge (map free-vars (syntax->list #'(expr ...))))]
+           [bindings (formals->boundmap #'formals)])
+       (filter (lambda (id)
+                 (not (bound-identifier-mapping-get bindings id (lambda () #f))))
+               free))]
+    [(case-lambda [formals expr ...] ...)
+     (merge (map free-vars (syntax->list
+                            #'((lambda formals expr ...) ...))))]
+    [(let-values ([(id ...) rhs] ...) expr ...)
+     (merge (cons (free-vars #'(lambda (id ... ...) expr ...))
+                  (map free-vars (syntax->list #'(rhs ...)))))]
+    [(letrec-values ([(id ...) rhs] ...) expr ...)
+     (free-vars #'(lambda (id ... ...) rhs ... expr ...))]      
+    [(_ expr ...)
+     ;; if, begin, begin0, set!, #%app, #%variable-reference, with-continuation-mark
+     (merge (map free-vars (syntax->list #'(expr ...))))]))
+