Background: Aneurysms of the A1 portion of the anterior cerebral artery are rare. The accessory middle cerebral artery is also a rare anomalous artery.
Case report: We operated on a 53-year-old man because of a giant aneurysm which arose at the junction of the accessory middle cerebral artery and the horizontal portion of the anterior cerebral artery (A1 portion).
Conclusion: This is the first report of a giant aneurysm of that region. A detailed evaluation of the angiogram is necessary prior to the operation, in order to select the most appropriate operative method to secure the blood flow of the accessory middle cerebral artery and distal anterior cerebral artery.