Thirty (2.5%) of 1200 consecutive patients with a first stroke had a spontaneous dissection with occlusion of the cervical internal carotid artery (ICA). A suggestive picture with ipsilateral headache and oculosympathetic paresis was uncommon (17%), so that diagnosis was uncertain before angiography. Seven patients died within one week. During follow-up (mean, 3.2 years) with sequential Doppler ultrasonographic testing, 12 survivors had a good recovery and early reopening of the occluded ICA, and 11 had a poor recovery usually without reopening of the ICA. Recurrence of a dissection occurred in only one patient. Large infarcts causing death or a severe disability were associated with an ICA thrombus and distal emboli; the organization of this intraluminal thrombosis may explain the absence of reopening in these cases while resorption of the intramural hematoma developed. Early heparin sodium therapy may help prevent intraluminal clotting without carrying an important risk of extending the dissection, but its clinical benefit remains unproven. Contrary to current opinions, ICA dissection with occlusion causing cerebral infarction may often carry a severe prognosis.