diff --git a/src/riemann/streams.clj b/src/riemann/streams.clj
index 4873a29c9..5e020a7a9 100644
--- a/src/riemann/streams.clj
+++ b/src/riemann/streams.clj
@@ -298,25 +298,30 @@
 
   Events without times accrue in the current window."
   [n & children]
-  (let [cutoff (ref 0)
-        buffer (ref [])]
+  (let [state (atom {:cutoff 0
+                     :buffer []
+                     :send? true})]
     (fn stream [event]
-      (let [events (dosync
-                     ; Compute minimum allowed time
-                     (let [cutoff (alter cutoff max (- (get event :time 0) n))]
-                       (when (or (nil? (:time event))
-                                 (< cutoff (:time event)))
-                         ; This event belongs in the buffer, and our cutoff may
-                         ; have changed.
-                         (alter buffer conj event)
-                         (alter buffer
-                                (fn alter [events]
-                                  (vec (filter
-                                         (fn [e] (or (nil? (:time e))
-                                                     (< cutoff (:time e))))
-                                         events)))))))]
-        (when events
-          (call-rescue events children))))))
+      (let [result (swap!
+                    state
+                    (fn [{:keys [cutoff buffer]}]
+                      ; Compute minimum allowed time
+                      (let [cutoff (max cutoff (- (get event :time 0) n))
+                            send? (or (nil? (:time event))
+                                      (< cutoff (:time event)))
+                            buffer (if send?
+                                        ; This event belongs in the buffer,
+                                        ; and our cutoff may have changed.
+                                     (vec (filter
+                                           (fn [e] (or (nil? (:time e))
+                                                       (< cutoff (:time e))))
+                                           (conj buffer event)))
+                                     buffer)]
+                        {:cutoff cutoff
+                         :buffer buffer
+                         :send? send?})))]
+        (when (:send? result)
+          (call-rescue (:buffer result) children))))))
 
 (defn- fixed-time-window-fn
   "A fixed window over the event stream in time. Emits vectors of events, such