✓ Twenty patients with posterior communicating artery aneurysms were treated with common carotid ligation. Postligation visualization was accomplished in 16 cases by ipsilateral brachial angiography. Two other aneurysms were visualized by contralateral brachial and contralaterial carotid angiography. The size of the aneurysm was measured before and after ligation. The relationship of postoperative reduction in size to preoperative angiographic characteristics was studied. Reduction in the size of the aneurysm occurred most often when preoperative angiography showed that the sac was long and its neck narrow, and when there was stasis of contrast material in the aneurysm.