diff -r e5149789e4ea Doc/library/asyncio-eventloop.rst
--- a/Doc/library/asyncio-eventloop.rst	Sun Apr 17 20:31:51 2016 +0300
+++ b/Doc/library/asyncio-eventloop.rst	Mon Apr 18 14:50:40 2016 -0400
@@ -634,6 +634,11 @@
 pool of processes). By default, an event loop uses a thread pool executor
 (:class:`~concurrent.futures.ThreadPoolExecutor`).
 
+.. versionchanged:: 3.5
+    :meth:`BaseEventLoop.run_in_executor` no longer configures the `max_workers` of the
+    thread pool executor it creates, instead leaving it up to the thread pool executor
+    (:class:`~concurrent.futures.ThreadPoolExecutor`) to set the default.
+
 .. coroutinemethod:: BaseEventLoop.run_in_executor(executor, func, \*args)
 
    Arrange for a *func* to be called in the specified executor.
diff -r e5149789e4ea Lib/asyncio/base_events.py
--- a/Lib/asyncio/base_events.py	Sun Apr 17 20:31:51 2016 +0300
+++ b/Lib/asyncio/base_events.py	Mon Apr 18 14:50:40 2016 -0400
@@ -43,7 +43,8 @@
 __all__ = ['BaseEventLoop']
 
 
-# Argument for default thread pool executor creation.
+# Argument for default thread pool executor creation. Only used on Python 3.4
+# and earlier.
 _MAX_WORKERS = 5
 
 # Minimum number of _scheduled timer handles before cleanup of
@@ -544,7 +545,12 @@
         if executor is None:
             executor = self._default_executor
             if executor is None:
-                executor = concurrent.futures.ThreadPoolExecutor(_MAX_WORKERS)
+                # Python 3.5 ThreadPoolExecutor has its own default for
+                # max_workers so use that.
+                if compat.PY35:
+                    executor = concurrent.futures.ThreadPoolExecutor()
+                else:
+                    executor = concurrent.futures.ThreadPoolExecutor(_MAX_WORKERS)
                 self._default_executor = executor
         return futures.wrap_future(executor.submit(func, *args), loop=self)