diff --git a/src/ahttpx/_request.py b/src/ahttpx/_request.py
index 6731259..78b8228 100644
--- a/src/ahttpx/_request.py
+++ b/src/ahttpx/_request.py
@@ -53,6 +53,17 @@ def __init__(
             elif content_length > 0:
                 self.headers = self.headers.copy_set("Content-Length", str(content_length))
 
+        elif method in ("POST", "PUT", "PATCH"):
+            # https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.2
+            # RFC 7230, Section 3.3.2, Content Length.
+            #
+            # A user agent SHOULD send a Content-Length in a request message when no
+            # Transfer-Encoding is sent and the request method defines a meaning for
+            # an enclosed payload body. For example, a Content-Length header field is
+            # normally sent in a POST request even when the value is 0.
+            # (indicating an empty payload body).
+            self.headers = self.headers.copy_set("Content-Length", "0")
+
     @property
     def body(self) -> bytes:
         if not hasattr(self, '_body'):
diff --git a/src/httpx/_request.py b/src/httpx/_request.py
index 0d5c8a6..1b739b1 100644
--- a/src/httpx/_request.py
+++ b/src/httpx/_request.py
@@ -53,6 +53,17 @@ def __init__(
             elif content_length > 0:
                 self.headers = self.headers.copy_set("Content-Length", str(content_length))
 
+        elif method in ("POST", "PUT", "PATCH"):
+            # https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.2
+            # RFC 7230, Section 3.3.2, Content Length.
+            #
+            # A user agent SHOULD send a Content-Length in a request message when no
+            # Transfer-Encoding is sent and the request method defines a meaning for
+            # an enclosed payload body. For example, a Content-Length header field is
+            # normally sent in a POST request even when the value is 0.
+            # (indicating an empty payload body).
+            self.headers = self.headers.copy_set("Content-Length", "0")
+
     @property
     def body(self) -> bytes:
         if not hasattr(self, '_body'):
diff --git a/tests/test_request.py b/tests/test_request.py
index c7a8db7..a69e1d1 100644
--- a/tests/test_request.py
+++ b/tests/test_request.py
@@ -64,3 +64,16 @@ def test_request_json():
         "Content-Type": "application/json",
     }
     assert r.read() == b'{"msg":"Hello, world"}'
+
+
+def test_request_empty_post():
+    r = httpx.Request("POST", "https://example.com")
+
+    assert repr(r) == "<Request [POST 'https://example.com']>"
+    assert r.method == "POST"
+    assert r.url == "https://example.com"
+    assert r.headers == {
+        "Host": "example.com",
+        "Content-Length": "0",
+    }
+    assert r.read() == b''