We studied 36 patients (21 women and 15 men) with spontaneous dissection of the internal carotid arteries. The ages of these patients ranged from 21 to 63 years. Focal unilateral headache was the most common symptom. Other common clinical manifestations (in decreasing order of frequency) included focal cerebral ischemic symptoms, oculosympathetic paresis, bruits, light-headedness, and neck pain. Less common symptoms were syncope, amaurosis fugax, scalp tenderness, swelling in the neck, and dysgeusia. Common angiographic manifestations (in decreasing order of frequency) were elongated, irregular, and frequently tapered narrowing of the lumen; abrupt luminal reconstitution (often at the carotid canal); aneurysms; intimal flaps; slow internal carotid artery--middle cerebral artery flow; tapered occlusion; and distal branch occlusions. The incidence of hypertension in these patients was considerably higher than that in the general population. Angiographic evidence of fibromuscular dysplasia was found in 14% of the patients, but atherosclerotic changes were uncommon. Follow-up ranged from 14 to 140 months (mean, 58.5 months). Twenty-three patients with 29 dissected internal carotid arteries were also restudied angiographically. The stenosis of the internal carotid artery either completely resolved or substantially improved in more than 85% of the dissected vessels. About two-thirds of the dissecting aneurysms either resolved or decreased in size. Clinically more than 85% of the patients had an excellent or complete recovery. Recurrence of the dissection or rupture of a dissecting aneurysm was not noted. Despite their disconcerting appearance on angiography, spontaneous dissections of the internal carotid arteries are often associated with a good prognosis.