OBJECTIVE: Aneurysms arising from the trunk of the supraclinoid internal carotid artery(ICA) unrelated to the branching are rare and treatment of these aneurysm is well known as difficult due to intraoperative premature rupture and postoperative bleeding. The authors report an analysis of 12 cases of ICA trunk aneurysms treated with transcranial or endovascular methods. METHODS: Between Mar. 1999 and Apr., 2001, 12 patients were treate for the ICA trunk aneurysms. The medical records and neuroimaging studies of the patients were reviewed retrospectively. RESULTS: These 11(92%) presented with symptoms related to intracranial hemorrhage. All aneurysms were small in size. Eleven patients(92%) were in good neurological status(Hunt Hess grade 0-III), one (8%) in poor grade(H-H grade IV). Eight patients were treated with transcranial surgery, operated on by the ipsilateral pterional approach and 4 patients were treated with endovascular surgery. Intraoperative premature rupture of the aneurysm occured in 6 cases(75%) with transcranial surgery and these were managed with ICA trapping in 4 cases, wrapping using temporalis fascia and clip in 1 and clipping with Sundt-Kee encircling clip in 1. The management outcome of the transcranial surgery was: 3(38%) Glasgow outcome scale(GOS) I, 1(12.5%) GOS II, 2(25%) GOS IV, and 2(25%) GOS V(death). The major causes of morbidity and mortality related to transcranial surgery were cerebral infarction and brain swelling due to intraoperative rupture and vasospasm. Four cases underwent endovascular surgery using Guglielmi detachable coils. The management outcome of the endovascular surgery was: 3(75%) GOS I and one GOS V 1(25%). The cause of mortality related to endosaccular coiling was brain swelling due to vasospasm. CONCLUSION: Treatment of the ICA trunk aneurysm, especially with blister-like nature, has high risk of the intraoperative rupture. Prevention and successful management of the intraoperative bleeding can be made by careful preoperative diagnosis of the aneurysm and preparation for the intraoperative rupture.