Fourteen patients with a spontaneous dissecting aneurysm of an internal carotid artery (ICA) have been admitted in our service since 1979 (incidence of 0,5 per 100,000 inhabitants per year). All these patients have been prospectively followed in order to determine their functional prognosis and a possible recanalization of the ICA. Three patients quickly died from an extensive middle cerebral artery infarct with brainstem compression. Among the 11 survivors, 7 completely recovered their functional ability or were left with very minor sequelae, and could go back to work. The 4 other patients remained with severe sequelae and could not work anymore. In the latter patients Doppler ultrasonographic study showed a persisting occlusion of the ICA, whereas a complete recanalization occurred in the former 7, usually as soon as the first month, on anticoagulant treatment. The presence on admission of a minor or moderate neurological deficit, a normal state of consciousness, an age above 45 years, and a patent collateral circulation allows to predict a favorable evolution (p less than 0.05). The same is true for the development of a partial or complete recanalization of the ICA on Doppler ultrasonography performed 2 to 4 weeks after admission. Sex, angiographic aspects of the dissection, and occurrence of headache or warning transient ischemic attacks had no prognostic significance in our study. Spontaneous dissecting aneurysms of the ICA can be a very serious disease, though nearly 50 p. 100 of the patients completely recover during the first months. Immediate anticoagulant therapy is still the treatment of choice and surgery is indicated only in those patients with recurrent episodes or a progressing stroke despite adequate anticoagulation.