From 1980 through 1982, intravenous extracranial digital subtraction angiography (DSA) was performed in 6684 patients at the Cleveland Clinic. Of these, 290 previously unoperated patients had asymptomatic carotid stenosis exceeding 50% of lumen diameter on unequivocal DSA studies. Either the presence or the absence of carotid bruits substantially misrepresented the severity of angiographic stenosis on approximately 30% of sides. Nonoperative management was employed in 195 patients, including 104 (53%) who received antiplatelet therapy, while another group of 95 patients underwent prophylactic carotid endarterectomy. During mean follow-up intervals of 33-38 months, surgical treatment significantly reduced the cumulative incidence of subsequent neurologic events in men (p = 0.05). Statistically unconfirmed trends also suggested that carotid endarterectomy tended to prevent late strokes in subsets of patients with greater than 70% stenosis or bilateral carotid lesions. The overall stroke rate for women was higher in the surgical group (p = 0.03), in part because of their unusual risk for perioperative complications (9%) in this particular series.