A 34-year-old Caucasian man presented with subarachnoid hemorrhage. Angiography revealed bilateral carotid occlusion at the cavernous sinus and an aneurysm at the basilar artery bifurcation. The whole brain was supplied with blood from the basilar artery and posterior cerebral arteries through a large number of collateral vessels to the internal carotid artery bifurcation, middle cerebral and anterior cerebral arteries: the moyamoya phenomenon. The aneurysm was clipped within hours of the subarachnoid hemorrhage. The relation between moyamoya disease and basilar artery aneurysms is discussed and some surgical and management considerations are given.