diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 750102f..9f9a582 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -19,7 +19,7 @@ dev
 
 **Bugfixes**
 
--
+- Recalculate the body length when serializing an ExtensionFrame.
 
 6.1.0 (2025-01-22)
 ------------------
diff --git a/src/hyperframe/frame.py b/src/hyperframe/frame.py
index 5bab673..3a51eed 100644
--- a/src/hyperframe/frame.py
+++ b/src/hyperframe/frame.py
@@ -905,6 +905,8 @@ def serialize(self) -> bytes:
         user code: it exists only as a helper method if frames need to be
         reconstituted.
         """
+        self.body_len = len(self.body)
+
         # Build the frame header.
         # First, get the flags.
         flags = self.flag_byte
diff --git a/tests/test_frames.py b/tests/test_frames.py
index b683354..60c2872 100644
--- a/tests/test_frames.py
+++ b/tests/test_frames.py
@@ -995,3 +995,16 @@ class TestExtensionFrame:
     def test_repr(self):
         f = ExtensionFrame(0xFF, 1, 42, b'hello')
         assert repr(f).endswith("type=255, flag_byte=42, body=<hex:68656c6c6f>")
+
+
+@pytest.mark.parametrize('body', [b'', b'hello', b'x' * 256])
+def test_extension_frame_serialized_body_length(body):
+    frame = ExtensionFrame(0xFF, 1, 42, body)
+    decoded = decode_frame(frame.serialize())
+    assert decoded.body == body
+    assert decoded.body_len == len(body)
+    assert decoded.flag_byte == 42
+    frame.body = b'replacement'
+    decoded = decode_frame(frame.serialize())
+    assert decoded.body == b'replacement'
+    assert decoded.body_len == len(frame.body)