diff --git a/lib/set.rb b/lib/set.rb
index 5a96c81832..e7d1be4f9f 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -464,7 +464,15 @@ class Set
   #     Set['a', 'b', 'z'] & ['a', 'b', 'c']    #=> #<Set: {"a", "b"}>
   def &(enum)
     n = self.class.new
-    do_with_enum(enum) { |o| n.add(o) if include?(o) }
+    if enum.is_a?(Set)
+      if enum.size > size
+        each { |o| n.add(o) if enum.include?(o) }
+      else
+        enum.each { |o| n.add(o) if include?(o) }
+      end
+    else
+      do_with_enum(enum) { |o| n.add(o) if include?(o) }
+    end
     n
   end
   alias intersection &