Summary:Nontraumatic intracranial arterial dissection was previously and is probably at present uncritically accepted as “cerebral infarction in otherwise healthy young adults.” This article challenges this conception of intracranial dissection. Data on 260 cases were collected from the literature (162 cases), unpublished cases in Japan (54 cases), and our own series (44 cases) to elucidate the natural history of nontraumatic intracranial dissections. The true incidence is not known. The incidence of symptomatic dissection is much higher in the vertebrobasilar than in the carotid system. The mean age of all the patients was 44.8 years. Patients with carotid lesions were younger (mean age, 32.2 years). The male/female ratio was 2:1. The most commonly affected artery was the vertebral, followed by the basilar and then the internal carotid arteries. Fifty-six percent of the patients were first seen with subarachnoid hemorrhage. Surgery was performed in 29% of the patients with carotid lesions and 60% of the patients with vertebrobasilar lesions. Dissection between the intima and media tends to cause arterial narrowing and occlusion, and dissection in the media or between the media and adventitia tends to cause rupture. Three fourths of the patients achieved favorable outcomes, and one fourth died. Outcome was better in the vertebrobasilar than in the carotid lesions.