PURPOSE: The author has studied the clinical results of CT and MR findings of proximal carotid artery occlusion using detachable balloons in the treatement of unclippable internal carotid (IC) aneurysms. MATERIALS AND METHODS: From 1987 to 1995, twenty-eight patients with IC aneurysms were treated by proximal arteryocclusion with detachable balloons. Of these patients, 4 had aneurysms arising from the supraclinoid portion ofthe IC artery, 23 had aneuryms arising from cavernous portion of the IC artery, and one had aneurysm arising fromcervical portion of the IC artery. Of the 28 patients, 7 patients without CT or MR examinations were excluded inthis study. The mean follow-up period was 18.6 months. The causes of aneurysm formation were spontaneous in 17cases and traumatic in 4 cases. RESULTS: Of 20 patients with aneurysms arising from supraclinoid and cavernousportion of the IC artery, 16 patients (80%) had cranial nerve symptoms by mass effect. Five patients had epistaxis(3 patient), carotid cavernous fistula (1 patient) or subarachnoid hemorrhage (1 patient) due to aneurysmrupture. Two patients, each with aneuryms arising from supraclinoid and cervical portion of carotid artery had 9thand 12th cranial nerve symptom. There were three instances of complication after permanent occlusion; two patientshad subarachnoid and intracerebral hemorrhage by aneurysm rupture and expired. One patient had ischemia ofposterior cerebral artery teritorry after one day. Delayed ischemic event did not occur during the follow-upperiod. All aneurysms of the carotid artery below the level of ophthalmic artery presented radiographic proof ofcomplete thrombosis within two months. However, complete thrombosis of aneurysm was considerably delayed in twoaneurysms arising from the supraclinoid portion of the carotid artery. In long-term follow-up study, complete-lythrombosed aneurysms decreased in size slowly. But incompletely thrombosed aneurysms did not decrease in size fora long time and began to contract after formation of complete thrombosis. All three traumatic pseudoaneurymscharacteristically decreased in size rapidly, comparing with spontaneous aneurysms. CONCLUSION: In conclusion,proximal IC balloon occlusion for unclippable IC aneurysms is a convenient, safe, and effective way of producinganeurysm obliteration. Longer-term follow-up study is needed for incompletely thrombosed aneuryms after balloonocclusion of the proximal IC artery.