diff --git a/scrapy/trunk/docs/ref/downloader-middleware.rst b/scrapy/trunk/docs/ref/downloader-middleware.rst
index c2285320c..8665e78be 100644
--- a/scrapy/trunk/docs/ref/downloader-middleware.rst
+++ b/scrapy/trunk/docs/ref/downloader-middleware.rst
@@ -34,6 +34,9 @@ thus it's recommended to leave it always enabled. Those tasks are:
     * If the request method is ``POST`` and the ``Content-Type`` header is not
       set, then set it to ``'application/x-www-form-urlencoded'``, the `default
       Form content type`_.
+
+    * If the request contains a body and the ``Content-Length`` headers it not
+      set, then set it to the ``len(body)``.
     
 .. _default Form content type: http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1
 
diff --git a/scrapy/trunk/scrapy/contrib/downloadermiddleware/common.py b/scrapy/trunk/scrapy/contrib/downloadermiddleware/common.py
index 91d95496c..793ed85ed 100644
--- a/scrapy/trunk/scrapy/contrib/downloadermiddleware/common.py
+++ b/scrapy/trunk/scrapy/contrib/downloadermiddleware/common.py
@@ -17,4 +17,6 @@ class CommonMiddleware(object):
         request.headers.setdefault('Accept-Language', self.header_accept_language)
         if request.method == 'POST':
             request.headers.setdefault('Content-Type', 'application/x-www-form-urlencoded')
+            if request.body:
+                request.headers.setdefault('Content-Length', '%d' % len(request.body))