Dissecting aneurysms of the cerebral arteries are exceedingly rare. The literature discloses 221-19 cases recorded since 1915 when Turnbull1 published the first case. Clinically this condition is indistinguishable from cerebral thrombosis. Cerebral artery dissecting aneurysms have shown no increase in diameter in the involved areas such as is seen in classical dissecting aneurysms. Instead a dissecting hematoma, which splits the intima from the medial coat, has been noted in all reported cases. In view of the lack of external gross findings in the involved vessels, it is possible that similar cases have been overlooked at autopsy unless a detailed examination of the cerebral vessels was carried out. The etiologies of the dissection are multiple. Trauma, migraine, syphilitic arteritis, medial cystic necrosis, and congenital medial defects have been incriminated; in a few cases the etiology has remained obscure. The patients afflicted were young, of an age when cerebral thrombosis