diff --git a/lib/http/cookie.rb b/lib/http/cookie.rb
index 70336ad..8fcd4a7 100644
--- a/lib/http/cookie.rb
+++ b/lib/http/cookie.rb
@@ -386,6 +386,12 @@ class HTTP::Cookie
     @domain = @domain_name.hostname
   end
 
+  # Returns the domain, with a dot prefixed only if the domain flag is
+  # on.
+  def dot_domain
+    @for_domain ? '.' << @domain : @domain
+  end
+
   # Returns the domain attribute value as a DomainName object.
   attr_reader :domain_name
 
diff --git a/test/test_http_cookie.rb b/test/test_http_cookie.rb
index d0b43a4..fd00076 100644
--- a/test/test_http_cookie.rb
+++ b/test/test_http_cookie.rb
@@ -162,6 +162,7 @@ class TestHTTPCookie < Test::Unit::TestCase
 
     assert_equal 'example.com', cookie.domain
     assert cookie.for_domain?
+    assert_equal '.example.com', cookie.dot_domain
   end
 
   def test_parse_domain_no_dot
@@ -173,6 +174,7 @@ class TestHTTPCookie < Test::Unit::TestCase
 
     assert_equal 'example.com', cookie.domain
     assert cookie.for_domain?
+    assert_equal '.example.com', cookie.dot_domain
   end
 
   def test_parse_domain_none
@@ -184,6 +186,7 @@ class TestHTTPCookie < Test::Unit::TestCase
 
     assert_equal 'example.com', cookie.domain
     assert !cookie.for_domain?
+    assert_equal 'example.com', cookie.dot_domain
   end
 
   def test_parse_max_age