diff --git a/scribble-lib/scribble/blueboxes.rkt b/scribble-lib/scribble/blueboxes.rkt
index 9db4b17e..785d5df4 100644
--- a/scribble-lib/scribble/blueboxes.rkt
+++ b/scribble-lib/scribble/blueboxes.rkt
@@ -15,7 +15,14 @@
   [fetch-blueboxes-method-tags (->* (symbol?) (#:blueboxes-cache blueboxes-cache?)
                                     (listof method-tag?))]))
 
+
+;; offset : (or/c exact-nonnegative-integer? #f)
+;; tag-ht : (or/c valid-blueboxes-info? #f)
+;; mod-time : (or/c exact-nonnegative-integer? #f)
+(struct bluebox-info (blueboxes.rktd [offset #:mutable] [tag-ht #:mutable] [mod-time #:mutable]))
+
 (struct blueboxes-cache (info-or-paths method->tags) #:mutable)
+
 (define (make-blueboxes-cache
          populate?
          #:blueboxes-dirs
@@ -45,25 +52,26 @@
 (define (fetch-strs-for-single-tag tag cache)
   (populate-cache! cache)
   (for/or ([ent (in-list (blueboxes-cache-info-or-paths cache))])
-    (define offset+lens (hash-ref (list-ref ent 2) tag #f))
-    (cond
-      [offset+lens
-       (define lines
-         (apply
-          append
-          (for/list ([offset+len (in-list offset+lens)])
-            (define fn (list-ref ent 0))
-            (define offset (list-ref ent 1))
-            (call-with-input-file fn
-              (λ (port)
-                (port-count-lines! port)
-                (file-position port (+ (car offset+len) offset))
-                (for/list ([i (in-range (cdr offset+len))])
-                  (read-line port)))))))
+    (check-and-update-bluebox-info! ent)
+    (match ent
+      [(bluebox-info blueboxes.rktd offset tag-ht _)
+       (define offset+lens (hash-ref tag-ht tag #f))
        (cond
-         [(ormap eof-object? lines) #f]
-         [else lines])]
-      [else #f])))
+         [offset+lens
+          (define lines
+            (apply
+             append
+             (for/list ([offset+len (in-list offset+lens)])
+               (call-with-input-file blueboxes.rktd
+                 (λ (port)
+                   (port-count-lines! port)
+                   (file-position port (+ (car offset+len) offset))
+                   (for/list ([i (in-range (cdr offset+len))])
+                     (read-line port)))))))
+          (cond
+            [(ormap eof-object? lines) #f]
+            [else lines])]
+         [else #f])])))
 
 (define (fetch-blueboxes-method-tags sym #:blueboxes-cache [cache (make-blueboxes-cache #f)])
   (populate-cache! cache)
@@ -78,9 +86,9 @@
 
 (define (compute-methods-table lst)
   (define meth-ht (make-hash))
-  (for ([three-tuple (in-list lst)])
-    (match three-tuple
-      [`(,file-path ,i ,tag-ht)
+  (for ([a-bluebox-info (in-list lst)])
+    (match a-bluebox-info
+      [(bluebox-info blueboxes.rktd offset tag-ht mod-time)
        (for ([(tag val) (in-hash tag-ht)])
          (when (method-tag? tag)
            (define-values (class/intf meth) (get-class/interface-and-method tag))
@@ -93,29 +101,47 @@
    values
    (for*/list ([doc-dir-name (in-list blueboxes-dirs)])
      (define blueboxes.rktd (build-path doc-dir-name "blueboxes.rktd"))
-     (and (file-exists? blueboxes.rktd)
-          (call-with-input-file blueboxes.rktd
-            (λ (port)
-              (port-count-lines! port)
-              (define first-line (read-line port))
-              (define pos (file-position port))
-              (define desed 
-                (with-handlers ([exn:fail? (λ (x) 
-                                             (log-warning "Failed to deserialize ~a: ~a"
-                                                          x
-                                                          (exn-message x))
-                                             #f)])
-                  (define candidate (deserialize (read port)))
-                  (unless (valid-blueboxes-info? candidate)
-                    (error 'build-blueboxes-cache
-                           "blueboxes info didn't have the right shape: ~s"
-                           candidate))
-                  candidate))
-              (and desed
-                   (list blueboxes.rktd
-                         (+ (string->number first-line) pos)
-                         desed))))))))
+     (define a-bluebox-info (bluebox-info blueboxes.rktd #f #f #f))
+     (populate-bluebox-info! a-bluebox-info)
+     a-bluebox-info)))
 
+(define (check-and-update-bluebox-info! a-bluebox-info)
+  (match a-bluebox-info
+    [(bluebox-info blueboxes.rktd offset tag-ht mod-time)
+     (when (or (not mod-time)
+               (and (file-exists? blueboxes.rktd)
+                    (not (mod-time . = . (file-or-directory-modify-seconds blueboxes.rktd)))))
+       (populate-bluebox-info! a-bluebox-info))]))
+
+(define (populate-bluebox-info! a-bluebox-info)
+  (define blueboxes.rktd (bluebox-info-blueboxes.rktd a-bluebox-info))
+  (cond
+    [(file-exists? blueboxes.rktd)
+     (call-with-input-file blueboxes.rktd
+       (λ (port)
+         (port-count-lines! port)
+         (define first-line (read-line port))
+         (define pos (file-position port))
+         (define desed 
+           (with-handlers ([exn:fail? (λ (x) 
+                                        (log-warning "Failed to deserialize ~a: ~a"
+                                                     x
+                                                     (exn-message x))
+                                        #f)])
+             (define candidate (deserialize (read port)))
+             (unless (valid-blueboxes-info? candidate)
+               (error 'build-blueboxes-cache
+                      "blueboxes info didn't have the right shape: ~s"
+                      candidate))
+             candidate))
+         (set-bluebox-info-offset! a-bluebox-info (and desed (+ (string->number first-line) pos)))
+         (set-bluebox-info-tag-ht! a-bluebox-info desed)
+         (set-bluebox-info-mod-time! a-bluebox-info
+                                     (file-or-directory-modify-seconds blueboxes.rktd))))]
+    [else
+     (set-bluebox-info-offset! a-bluebox-info #f)
+     (set-bluebox-info-tag-ht! a-bluebox-info #f)
+     (set-bluebox-info-mod-time! a-bluebox-info #f)]))
 
 (define valid-blueboxes-info?
   (hash/c