A 44-year-old man experienced the sudden onset of horizontal diplopia and hemifacial numbness. Arteriography demonstrated a left intrapetrous carotid artery aneurysm. The patient was successfully treated with a left superficial temporal artery to middle cerebral artery bypass followed by balloon entrapment of the aneurysm. There have been at least 40 previously reported cases of aneurysms of the petrous portion of the carotid artery. These aneurysms can be mycotic, traumatic, or developmental in origin. They can present with massive otorrhagia or epistaxis from acute rupture or with decreased hearing and paresis of the fifth through eighth cranial nerves and, less frequently, of the ninth, 10th, and 12th cranial nerves caused by direct pressure. They can also produce pulsatile tinnitus, and sometimes they are discovered as a retrotympanic vascular mass during otological examination. The treatment of choice is carotid artery occlusion. Trapping of the aneurysm by detachable balloons eliminates immediately the risk of hemorrhage, offers the possibility of test occlusion of the internal carotid artery with the patient awake prior to permanent occlusion, and should also reduce the risk of thromboembolism. It should be preceded by a bypass procedure when preliminary evaluation indicates that the patient will not tolerate internal carotid artery occlusion.