diff --git a/Lib/http/server.py b/Lib/http/server.py
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -355,7 +355,7 @@
 
         """
         self.send_response_only(100)
-        self.flush_headers()
+        self.end_headers()
         return True
 
     def handle_one_request(self):
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -195,6 +195,9 @@
 - Issue #19286: Directories in ``package_data`` are no longer added to
   the filelist, preventing failure outlined in the ticket.
 
+- Issue #18574: Fixed handling of 100-continue reply from server in
+  http.client.HTTPConnection. Patch by Nikolaus Rath.
+
 IDLE
 ----