diff -r 9332a545ad85 Lib/test/test_urllib2.py
--- a/Lib/test/test_urllib2.py	Thu Mar 12 22:01:30 2015 +0200
+++ b/Lib/test/test_urllib2.py	Fri Mar 13 15:24:10 2015 -0700
@@ -12,10 +12,11 @@
 # The proxy bypass method imported below has logic specific to the OSX
 # proxy config data structure but is testable on all platforms.
 from urllib.request import Request, OpenerDirector, _parse_proxy, _proxy_bypass_macosx_sysconf
-from urllib.parse import urlparse
+from urllib.parse import urlparse, urlsplit
 import urllib.error
 import http.client
 
+
 # XXX
 # Request
 # CacheFTPHandler (hard to write)
@@ -290,7 +291,7 @@
 
 
 class MockHTTPClass:
-    def __init__(self):
+    def __init__(self, host, timeout=None):
         self.level = 0
         self.req_headers = []
         self.data = None
@@ -442,7 +443,7 @@
 
     def __init__(self):
         urllib.request.AbstractHTTPHandler.__init__(self)
-        self.httpconn = MockHTTPClass()
+        self.httpconn = MockHTTPClass('example.com')
 
     def https_open(self, req):
         return self.do_open(self.httpconn, req)
@@ -773,6 +774,25 @@
                 self.assertEqual(req.type, "ftp")
             self.assertEqual(req.type == "ftp", ftp)
 
+    def test_idna(self):
+        request_headers = {}
+        class MockHTTPConnection(MockHTTPClass):
+            def request(self, method, url, body, headers):
+                nonlocal request_headers
+                request_headers = headers
+
+        host = 'http://\u03b4\u03c0\u03b8.gr'
+        expected_host = urlsplit(host).netloc.encode('idna').decode('ascii')
+
+        handler = urllib.request.HTTPHandler()
+        handler.parent = MockOpener()
+        request = handler.http_request(Request(host))
+        request.timeout = None
+        handler.do_open(MockHTTPConnection, request)
+
+        self.assertIn('Host', request_headers)
+        self.assertEqual(request_headers['Host'], expected_host)
+
     def test_http(self):
 
         h = urllib.request.AbstractHTTPHandler()
@@ -783,7 +803,7 @@
             req = Request(url, data, {"Foo": "bar"})
             req.timeout = None
             req.add_unredirected_header("Spam", "eggs")
-            http = MockHTTPClass()
+            http = MockHTTPClass('example.com')
             r = h.do_open(http, req)
 
             # result attributes
diff -r 9332a545ad85 Lib/urllib/request.py
--- a/Lib/urllib/request.py	Thu Mar 12 22:01:30 2015 +0200
+++ b/Lib/urllib/request.py	Fri Mar 13 15:24:10 2015 -0700
@@ -1192,6 +1192,9 @@
                 del headers[proxy_auth_hdr]
             h.set_tunnel(req._tunnel_host, headers=tunnel_headers)
 
+        if 'Host' in headers:
+            headers['Host'] = headers['Host'].encode('idna').decode('ascii')
+
         try:
             try:
                 h.request(req.get_method(), req.selector, req.data, headers)
@@ -1791,6 +1794,8 @@
         headers["Connection"] = "close"
 
         for header, value in self.addheaders:
+            if header.lower() == 'host':
+                value = value.encode('idna').decode('ascii')
             headers[header] = value
 
         if data is not None: