Eighty-two cases of cerebrovascular moyamoya disease were studied by cerebral angiography and computerized tomography. Occlusive lesions were demonstrated not only in the anterior circulation but also in the posterior circulation, and they were associated with the development of an abnormal vascular network (moyamoya vessels). Although occlusive lesions do occur in the vertebrobasilar system, the vertebrobasilar system also acts as a source of collateral channels to the anterior circulation in this disease.