Seventy patients with spontaneous and 21 with traumatic extracranial internal carotid artery dissections were studied clinically and angiographically with mean follow-ups of 64 (spontaneous group) and 40 months (traumatic group). Sixty percent of the patients in the spontaneous group and 71% in the traumatic group also had follow-up angiograms. In traumatic dissections aneurysms were common, significantly fewer aneurysms resolved or became smaller and fewer stenoses resolved or improved, whereas more stenoses progressed to occlusion. Traumatic dissections were more likely to leave the patients with neurological deficits. A significantly higher percentage of the patients with spontaneous dissections were asymptomatic at follow-up compared with the traumatic group. Although both spontaneous and traumatic dissections of extracranial internal carotid arteries mostly carry a good prognosis, the outcome may be somewhat less favorable for the traumatic group.