diff --git a/tornado/stack_context.py b/tornado/stack_context.py
index 5e744815..df186999 100644
--- a/tornado/stack_context.py
+++ b/tornado/stack_context.py
@@ -74,6 +74,7 @@ import itertools
 import sys
 import threading
 
+from tornado.util import raise_exc_info
 
 class _State(threading.local):
     def __init__(self):
@@ -248,4 +249,4 @@ def _nested(*managers):
             # Don't rely on sys.exc_info() still containing
             # the right information. Another exception may
             # have been raised and caught by an exit method
-            raise exc[0], exc[1], exc[2]
+            raise_exc_info(exc)
diff --git a/tornado/test/runtests.py b/tornado/test/runtests.py
index fdedd438..8bf8eef9 100755
--- a/tornado/test/runtests.py
+++ b/tornado/test/runtests.py
@@ -24,6 +24,7 @@ TEST_MODULES = [
     'tornado.test.template_test',
     'tornado.test.testing_test',
     'tornado.test.twisted_test',
+    'tornado.test.util_test',
     'tornado.test.web_test',
     'tornado.test.wsgi_test',
 ]
diff --git a/tornado/test/util_test.py b/tornado/test/util_test.py
new file mode 100644
index 00000000..24f607a4
--- /dev/null
+++ b/tornado/test/util_test.py
@@ -0,0 +1,25 @@
+from __future__ import absolute_import, division, with_statement
+import sys
+import unittest
+
+from tornado.util import raise_exc_info
+
+class RaiseExcInfoTest(unittest.TestCase):
+    def test_two_arg_exception(self):
+        # This test would fail on python 3 if raise_exc_info were simply
+        # a three-argument raise statement, because TwoArgException
+        # doesn't have a "copy constructor"
+        class TwoArgException(Exception):
+            def __init__(self, a, b):
+                super(TwoArgException, self).__init__()
+                self.a, self.b = a, b
+
+        try:
+            raise TwoArgException(1, 2)
+        except TwoArgException:
+            exc_info = sys.exc_info()
+        try:
+            raise_exc_info(exc_info)
+            self.fail("didn't get expected exception")
+        except TwoArgException, e:
+            self.assertTrue(e is exc_info[1])
diff --git a/tornado/testing.py b/tornado/testing.py
index 5484e0eb..30fd87ec 100644
--- a/tornado/testing.py
+++ b/tornado/testing.py
@@ -32,6 +32,7 @@ except ImportError:
     HTTPServer = None
     IOLoop = None
 from tornado.stack_context import StackContext, NullContext
+from tornado.util import raise_exc_info
 import contextlib
 import logging
 import signal
@@ -142,12 +143,7 @@ class AsyncTestCase(unittest.TestCase):
         if self.__failure is not None:
             failure = self.__failure
             self.__failure = None
-            # 2to3 isn't smart enough to convert three-argument raise
-            # statements correctly in some cases.
-            if isinstance(failure[1], failure[0]):
-                raise failure[1], None, failure[2]
-            else:
-                raise failure[0], failure[1], failure[2]
+            raise_exc_info(failure)
 
 
     def run(self, result=None):
diff --git a/tornado/util.py b/tornado/util.py
index 23ffda1f..e19ca900 100644
--- a/tornado/util.py
+++ b/tornado/util.py
@@ -46,6 +46,24 @@ else:
     bytes_type = str
 
 
+def raise_exc_info(exc_info):
+    """Re-raise an exception (with original traceback) from an exc_info tuple.
+
+    The argument is a ``(type, value, traceback)`` tuple as returned by
+    `sys.exc_info`.
+    """
+    # 2to3 isn't smart enough to convert three-argument raise
+    # statements correctly in some cases.
+    if isinstance(exc_info[1], exc_info[0]):
+        raise exc_info[1], None, exc_info[2]
+        # After 2to3: raise exc_info[1].with_traceback(exc_info[2])
+    else:
+        # I think this branch is only taken for string exceptions,
+        # which were removed in Python 2.6.
+        raise exc_info[0], exc_info[1], exc_info[2]
+        # After 2to3: raise exc_info[0](exc_info[1]).with_traceback(exc_info[2])
+
+
 def doctests():
     import doctest
     return doctest.DocTestSuite()
diff --git a/tornado/web.py b/tornado/web.py
index ab617379..645eb68f 100644
--- a/tornado/web.py
+++ b/tornado/web.py
@@ -83,7 +83,7 @@ from tornado import locale
 from tornado import stack_context
 from tornado import template
 from tornado.escape import utf8, _unicode
-from tornado.util import b, bytes_type, import_object, ObjectDict
+from tornado.util import b, bytes_type, import_object, ObjectDict, raise_exc_info
 
 try:
     from io import BytesIO  # python 3
@@ -739,7 +739,7 @@ class RequestHandler(object):
                 kwargs['exception'] = exc_info[1]
                 try:
                     # Put the traceback into sys.exc_info()
-                    raise exc_info[0], exc_info[1], exc_info[2]
+                    raise_exc_info(exc_info)
                 except Exception:
                     self.finish(self.get_error_html(status_code, **kwargs))
             else:
@@ -984,7 +984,7 @@ class RequestHandler(object):
             # the exception value instead of the full triple,
             # so re-raise the exception to ensure that it's in
             # sys.exc_info()
-            raise type, value, traceback
+            raise_exc_info((type, value, traceback))
         except Exception:
             self._handle_request_exception(value)
         return True