diff -r e281a57d5b29 Lib/urllib/request.py
--- a/Lib/urllib/request.py	Tue Apr 19 08:53:14 2016 +0200
+++ b/Lib/urllib/request.py	Tue Apr 19 09:58:08 2016 +0200
@@ -2453,12 +2453,21 @@
     different way, you can pass a proxies dictionary to the
     [Fancy]URLopener constructor.
 
+    If both lowercase and uppercase variables exist (and disagree),
+    lowercase overrules uppercase.
     """
     proxies = {}
+    # in order to prioritize lowercase variables, 
+    # process environment in two passes
     for name, value in os.environ.items():
-        name = name.lower()
-        if value and name[-6:] == '_proxy':
-            proxies[name[:-6]] = value
+        if value and name[-6:] == '_PROXY':
+            proxies[name[:-6].lower()] = value
+    for name, value in os.environ.items():
+        if name[-6:] == '_proxy':
+            if value:
+                proxies[name[:-6]] = value
+            else:
+                proxies.pop(name[:-6], None)
     return proxies
 
 def proxy_bypass_environment(host):