diff --git a/pcapkit/protocols/misc/pcap/header.py b/pcapkit/protocols/misc/pcap/header.py
index 7cba752a40..66e38787c4 100644
--- a/pcapkit/protocols/misc/pcap/header.py
+++ b/pcapkit/protocols/misc/pcap/header.py
@@ -197,8 +197,8 @@ def make(self,
 
         Args:
             byteorder: header byte order
-            lilendian: little-endian flag
-            bigendian: big-endian flag
+            lilendian: little-endian flag; overrides ``byteorder`` if given
+            bigendian: big-endian flag; overrides ``byteorder`` if given
             nanosecond: nanosecond-resolution file flag
             version: version information
             version_major: major version number
@@ -336,21 +336,20 @@ def _make_magic(self, byteorder: 'Literal["big", "little"]' = sys.byteorder,
 
         Args:
             byteorder: header byte order
-            lilendian: little-endian flag
-            bigendian: big-endian flag
+            lilendian: little-endian flag; overrides ``byteorder`` if given
+            bigendian: big-endian flag; overrides ``byteorder`` if given
             nanosecond: nanosecond-resolution file flag
 
         Returns:
             Magic number and little-endian flag.
 
         """
-        if lilendian is not None and bigendian is not None:
-            if lilendian == bigendian:
-                raise EndianError('unresolved byte order')
-            if bigendian:
-                return _MAGIC_NUM[('big', nanosecond)], False
-            if lilendian:
-                return _MAGIC_NUM[('little', nanosecond)], True
+        if lilendian is not None and bigendian is not None and lilendian == bigendian:
+            raise EndianError('unresolved byte order')
+        if bigendian is not None:
+            byteorder = 'big' if bigendian else 'little'
+        elif lilendian is not None:
+            byteorder = 'little' if lilendian else 'big'
 
         if byteorder not in ('little', 'big'):
             raise EndianError(f"unknown byte order: {byteorder!r}")
diff --git a/tests/protocols/misc/pcap/test_header_frame_unit.py b/tests/protocols/misc/pcap/test_header_frame_unit.py
index e52a40bac5..3e82807029 100644
--- a/tests/protocols/misc/pcap/test_header_frame_unit.py
+++ b/tests/protocols/misc/pcap/test_header_frame_unit.py
@@ -146,8 +146,8 @@ class NamedBytesIO(io.BytesIO):
                          (b'\xa1\xb2\xc3\xd4', False))
         self.assertEqual(header._make_magic(lilendian=True, bigendian=False),
                          (b'\xd4\xc3\xb2\xa1', True))
-        self.assertEqual(header._make_magic(byteorder='big', lilendian='', bigendian=False),  # type: ignore[arg-type]
-                         (b'\xa1\xb2\xc3\xd4', False))
+        self.assertEqual(header._make_magic(byteorder='big', bigendian=False),
+                         (b'\xd4\xc3\xb2\xa1', True))
         with self.assertRaises(EndianError):
             header._make_magic(lilendian=True, bigendian=True)
         with self.assertRaises(EndianError):
diff --git a/tests/protocols/misc/pcap/test_header_lone_endian_flag_unit.py b/tests/protocols/misc/pcap/test_header_lone_endian_flag_unit.py
new file mode 100644
index 0000000000..4942417767
--- /dev/null
+++ b/tests/protocols/misc/pcap/test_header_lone_endian_flag_unit.py
@@ -0,0 +1,50 @@
+from __future__ import annotations
+
+import importlib.util
+import unittest
+
+from tests._support import reimport_once_per_class
+
+RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
+HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
+
+BIG = b'\xa1\xb2\xc3\xd4'
+LITTLE = b'\xd4\xc3\xb2\xa1'
+
+
+@unittest.skipUnless(HAS_RUNTIME, 'runtime dependencies not installed')
+class PCAPHeaderLoneEndianFlagUnitTests(unittest.TestCase):
+    """A single ``lilendian``/``bigendian`` flag selects the byte order (GH-1144)."""
+
+    def setUp(self) -> None:
+        reimport_once_per_class(self)
+
+    def test_lone_flag_overrides_byteorder(self) -> None:
+        from pcapkit.protocols.misc.pcap.header import Header
+
+        # ``byteorder`` is set to the opposite order, so a flag that is ignored
+        # shows up regardless of the host's own byte order
+        cases = (
+            ({'bigendian': True, 'byteorder': 'little'}, BIG, 'big'),
+            ({'bigendian': False, 'byteorder': 'big'}, LITTLE, 'little'),
+            ({'lilendian': True, 'byteorder': 'big'}, LITTLE, 'little'),
+            ({'lilendian': False, 'byteorder': 'little'}, BIG, 'big'),
+        )
+        for kwargs, magic, byteorder in cases:
+            with self.subTest(**kwargs):
+                header = Header(**kwargs)
+                self.assertEqual(header.data[:4], magic)
+                self.assertEqual(header.info.magic_number.byteorder, byteorder)
+
+    def test_conflicting_flags_are_rejected(self) -> None:
+        from pcapkit.protocols.misc.pcap.header import Header
+        from pcapkit.utilities.exceptions import EndianError
+
+        for flag in (True, False):
+            with self.subTest(flag=flag):
+                with self.assertRaises(EndianError):
+                    Header(lilendian=flag, bigendian=flag)
+
+
+if __name__ == '__main__':
+    unittest.main()