In a three-year study, the clinical course and results of intracranial angiography were compared in patients having an acute stroke in the carotid artery territory combined with angiographic abnormalities indicating severe extracranial carotid stenosis or occlusion. Two major mechanisms of stroke were delineated. In one group, the angiographic intracranial abnormalities strongly suggested the presence of embolism in the cerebral vessels supplied by the stenotic or occluded carotid artery; many of these patients had no obvious transient ischemic attacks prior to their stroke and experienced a moderate to severe clinical deficit. In the other group, evidence of embolism was absent; many showed a widespread delay in cerebral arterial perfusion, experienced a greater frequency of transient ischemic attacks before their stroke, and had a milder stroke than did those with embolism.