diff --git a/twisted/web/twcgi.py b/twisted/web/twcgi.py
--- a/twisted/web/twcgi.py
+++ b/twisted/web/twcgi.py
@@ -113,7 +113,7 @@ class CGIScript(resource.Resource):
         # Propogate HTTP headers
         for title, header in request.getAllHeaders().items():
             envname = string.upper(string.replace(title, '-', '_'))
-            if title not in ('content-type', 'content-length'):
+            if title not in ('content-type', 'content-length', 'proxy'):
                 envname = "HTTP_" + envname
             env[envname] = header
         # Propogate our environment