diff --git a/lib/rack/reverse_proxy.rb b/lib/rack/reverse_proxy.rb
index b7acfcf..aa195b2 100644
--- a/lib/rack/reverse_proxy.rb
+++ b/lib/rack/reverse_proxy.rb
@@ -20,7 +20,8 @@ def call(env)
       headers = Rack::Utils::HeaderHash.new
       env.each { |key, value|
         if key =~ /HTTP_(.*)/
-          headers[$1] = value
+          header = $1.gsub('_', '-')
+          headers[header] = value
         end
       }
       headers['HOST'] = uri.host if all_opts[:preserve_host]