diff --git a/lib/rate_limit.rb b/lib/rate_limit.rb
index 27d02ab..503c42d 100644
--- a/lib/rate_limit.rb
+++ b/lib/rate_limit.rb
@@ -1,16 +1,22 @@
 require 'thread'
 
 class RateLimit
+  # Every Nth allow? call also drops empty keys, so IPs that stop posting
+  # don't accumulate in @hits forever.
+  SWEEP_EVERY = 100
+
   def initialize(limit:, window:)
     @limit = limit
     @window = window
     @hits = Hash.new { |h, k| h[k] = [] }
     @mutex = Mutex.new
+    @calls = 0
   end
 
   def allow?(key)
     now = Time.now.to_i
     @mutex.synchronize do
+      prune(now - @window) if (@calls += 1) % SWEEP_EVERY == 0
       @hits[key].reject! { |t| t < now - @window }
       if @hits[key].size >= @limit
         false
@@ -22,10 +28,14 @@ class RateLimit
   end
 
   def sweep!
-    cutoff = Time.now.to_i - @window
-    @mutex.synchronize do
-      @hits.each_value { |arr| arr.reject! { |t| t < cutoff } }
-      @hits.delete_if { |_, arr| arr.empty? }
-    end
+    @mutex.synchronize { prune(Time.now.to_i - @window) }
+  end
+
+  private
+
+  # Caller must hold @mutex.
+  def prune(cutoff)
+    @hits.each_value { |arr| arr.reject! { |t| t < cutoff } }
+    @hits.delete_if { |_, arr| arr.empty? }
   end
 end