diff --git a/aiohttp/client_reqrep.py b/aiohttp/client_reqrep.py
index d536c0a1ca4..c063c56314e 100644
--- a/aiohttp/client_reqrep.py
+++ b/aiohttp/client_reqrep.py
@@ -3,6 +3,7 @@
 import contextlib
 import functools
 import io
+import itertools
 import re
 import sys
 import traceback
@@ -244,6 +245,9 @@ class ClientRequest:
     }
     POST_METHODS = {hdrs.METH_PATCH, hdrs.METH_POST, hdrs.METH_PUT}
     ALL_METHODS = GET_METHODS.union(POST_METHODS).union({hdrs.METH_DELETE})
+    _HOST_STRINGS = frozenset(
+        map("".join, itertools.product(*zip("host".upper(), "host".lower())))
+    )
 
     DEFAULT_HEADERS = {
         hdrs.ACCEPT: "*/*",
@@ -470,7 +474,7 @@ def update_headers(self, headers: Optional[LooseHeaders]) -> None:
 
         for key, value in headers:  # type: ignore[misc]
             # A special case for Host header
-            if key.lower() == "host":
+            if key in self._HOST_STRINGS:
                 self.headers[key] = value
             else:
                 self.headers.add(key, value)