diff --git a/grequests.py b/grequests.py
index 0ceab0f..f4ec5c7 100755
--- a/grequests.py
+++ b/grequests.py
@@ -10,6 +10,7 @@
 """
 from functools import partial
 import traceback
+
 try:
     import gevent
     from gevent import monkey as curious_george
@@ -22,7 +23,6 @@
 
 from requests import Session
 
-
 __all__ = (
     'map', 'imap',
     'get', 'options', 'head', 'post', 'put', 'patch', 'delete', 'request'
@@ -47,6 +47,9 @@ def __init__(self, method, url, **kwargs):
         self.session = kwargs.pop('session', None)
         if self.session is None:
             self.session = Session()
+            self._close = True
+        else:
+            self._close = False  # don't close adapters after each request if the user provided the session
 
         callback = kwargs.pop('callback', None)
         if callback:
@@ -73,6 +76,11 @@ def send(self, **kwargs):
         except Exception as e:
             self.exception = e
             self.traceback = traceback.format_exc()
+        finally:
+            if self._close:
+                # if we provided the session object, make sure we're cleaning up
+                # because there's no sense in keeping it open at this point if it wont be reused
+                self.session.close()
         return self