Spontaneous dissection of a major subarachnoid artery is an uncommon cause of stroke in young and apparently healthy individuals. Such dissection does not correlate well with systemic conditions such as emboli, thrombotic or hemorrhagic tendencies or hypertension, nor with disease of blood vessels such as atherosclerosis, fibromuscular dysplasia, cystic medial necrosis or Moya-Moya disease. Unusual exertion has been implicated as a factor in some cases. Gap defects were found in the internal elastic lamina near the site of dissection in three of four cases of spontaneous dissection reported here. It is suggested that such defects, because of their unusual size or number, may be responsible for initiating dissection in susceptible individuals. A detailed study of the major cerebral arteries in cases of dissection and control cases to document the size and frequency of such defects could shed light on the pathogenesis of spontaneous dissection. The subsequent course of events tends to differ in the internal carotid and vertebro-basilar systems. In most cases, the dissection in the internal carotid system is sub-intimal leading to thrombosis and cerebral infarction, while in the vertebrobasilar system dissection lies between the media and adventitia leading to subarachnoid hemorrhage. It is not known what structural differences of the two arterial systems may underlie this difference in the pattern of arterial dissection.