diff --git a/httpx/_transports/wsgi.py b/httpx/_transports/wsgi.py
index 8592ffe017..7a63c0a367 100644
--- a/httpx/_transports/wsgi.py
+++ b/httpx/_transports/wsgi.py
@@ -119,6 +119,7 @@ def handle_request(self, request: Request) -> Response:
         seen_status = None
         seen_response_headers = None
         seen_exc_info = None
+        seen_output: typing.List[bytes] = []
 
         def start_response(
             status: str,
@@ -129,9 +130,11 @@ def start_response(
             seen_status = status
             seen_response_headers = response_headers
             seen_exc_info = exc_info
-            return lambda _: None
+            return seen_output.append
 
         result = self.app(environ, start_response)
+        if seen_output:
+            result = itertools.chain(seen_output, result)
 
         stream = WSGIByteStream(result)
 
diff --git a/tests/test_wsgi.py b/tests/test_wsgi.py
index 0134bee854..29d7fe4838 100644
--- a/tests/test_wsgi.py
+++ b/tests/test_wsgi.py
@@ -64,6 +64,22 @@ def output_generator(f: typing.IO[bytes]) -> typing.Iterator[bytes]:
     return output_generator(f=environ["wsgi.input"])
 
 
+def echo_body_with_write_callable(
+    environ: "WSGIEnvironment", start_response: "StartResponse"
+) -> typing.Iterable[bytes]:
+    status = "200 OK"
+    output = environ["wsgi.input"].read()
+
+    response_headers = [
+        ("Content-type", "text/plain"),
+    ]
+
+    write = start_response(status, response_headers)
+    write(output)
+
+    return (b" and ", b"more")
+
+
 def raise_exc(
     environ: WSGIEnvironment,
     start_response: StartResponse,
@@ -115,6 +131,13 @@ def test_wsgi_upload_with_response_stream():
     assert response.text == "example"
 
 
+def test_wsgi_upload_with_write_callable():
+    client = httpx.Client(app=echo_body_with_write_callable)
+    response = client.post("http://www.example.org/", content=b"example")
+    assert response.status_code == 200
+    assert response.text == "example and more"
+
+
 def test_wsgi_exc():
     transport = httpx.WSGITransport(app=raise_exc)
     client = httpx.Client(transport=transport)