diff --git a/tests/http_server.py b/tests/http_server.py
index bee754e..a80f172 100644
--- a/tests/http_server.py
+++ b/tests/http_server.py
@@ -131,8 +131,8 @@ async def on_request(**server):
 async def on_response(content_type=b'text/plain', **server):
     response = server['response']
 
-    assert response.headers[b'x-foo'] == [b'X-Foo: bar']
-    assert b'Set-Cookie: sess=www; ' in response.headers[b'set-cookie'][0]
+    assert response.headers[b'x-foo'] == [b'bar']
+    assert b'sess=www; ' in response.headers[b'set-cookie'][0]
 
     response.set_header(b'X-Foo', b'baz')
 
diff --git a/tremolo/lib/http_response.py b/tremolo/lib/http_response.py
index e4bfd8d..615fa67 100644
--- a/tremolo/lib/http_response.py
+++ b/tremolo/lib/http_response.py
@@ -24,6 +24,14 @@
 CONNECTIONS = (b'close', b'keep-alive', b'upgrade')
 
 
+class HeaderKey(bytes):
+    def __new__(cls, name):
+        obj = super().__new__(cls, name.lower())
+        obj.name = name  # original casing
+
+        return obj
+
+
 class HTTPResponse(Response):
     __slots__ = ('line', 'content_type', 'http_chunked', '_headers')
 
@@ -59,12 +67,12 @@ def append_header(self, name, value):
         if b'\r' in name or b'\n' in name or b'\r' in value or b'\n' in value:
             raise InternalServerError
 
-        key = name.lower()
+        key = HeaderKey(name)
 
         if key in self.headers:
-            self.headers[key].append(name + b': ' + value)
+            self.headers[key].append(value)
         else:
-            self.headers[key] = [name + b': ' + value]
+            self.headers[key] = [value]
 
     def set_header(self, name, value=''):
         if isinstance(name, str):
@@ -76,7 +84,7 @@ def set_header(self, name, value=''):
         if b'\r' in name or b'\n' in name or b'\r' in value or b'\n' in value:
             raise InternalServerError
 
-        self.headers[name.lower()] = [name + b': ' + value]
+        self.headers[HeaderKey(name)] = [value]
 
     def set_base_headers(self):
         self.set_header(
@@ -179,8 +187,9 @@ async def end(self, data=b'', *, keepalive=True, **kwargs):
                  self.content_type,
                  content_length,
                  CONNECTIONS[keepalive and self.request.http_keepalive],
-                 b'\r\n'.join(b'\r\n'.join(v) for k, v in self.headers.items()
-                              if k not in excludes),
+                 b'\r\n'.join(k.name + b': ' + value for k in self.headers
+                              if k not in excludes
+                              for value in self.headers[k]),
                  data), **kwargs
             )
             self.headers_sent(True)
@@ -245,7 +254,8 @@ async def write(self, data=None, *, chunked=None, buffer_size=16384,
 
             await self.send(
                 b' '.join(self.line) + b'\r\n' +
-                b'\r\n'.join(b'\r\n'.join(v) for v in self.headers.values()) +
+                b'\r\n'.join(k.name + b': ' + value for k in self.headers
+                             for value in self.headers[k]) +
                 b'\r\n\r\n'
             )
             self.headers_sent(True)