diff --git a/requests/models.py b/requests/models.py
index 9198c4c3..a845b44e 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -387,6 +387,8 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         self.body = body
 
     def prepare_content_length(self, body):
+        if self.headers.get('Content-Length'):
+            return
         if hasattr(body, 'seek') and hasattr(body, 'tell'):
             body.seek(0, 2)
             self.headers['Content-Length'] = str(body.tell())