A six year old girl complained of sudden severe headache, became hemiplegic and unconscious. A right carotid arteriogram revealed an obstruction of the right anterior cerebral artery and many sulvian branches. Death occurred four days later. At autopsy, a recent softening of nearly all the right middle cerebral arterial territory was found. Thrombus filled the sylvian artery and its main branches. Histologic examination of the vessel walls showed a dissecting infiltration of blood between the internal elastic lamina and the media. This particular form of dissecting aneurysm, occurring in young subjects, in the absence of atherosclerosis, high blood pressure and idiopathic medial necrosis, represents a distinct medial necrosis, represents a distinct nosologic entity that has been called "Obstructive parietal hemodissection of intracranial vessels." The pathogenesis of the disease is unknown: trauma has been mentioned, also congenital defects in the elastic lamina or other morphologic abnormalities of that lamina.