To define the natural history of the asymptomatic, nonstenotic, ulcerative lesion involving the carotid artery bifurcation, the arteriograms and clinical course of 67 patients with 72 asymptomatic ulcerative lesions of the carotid artery were reviewed. The angiographic appearance of ulceration was classified into three groups: minimal (group A), large (group B), and compound (group C). Using life-table methods, the clinical course of these patients was compared between groups and was also compared to a nonrandomized surgically treated group of patients with nonstenotic ulcerative lesions in whom operation was performed for hemispheric or monocular symptoms. There were no significant (P greater than .1) differences in mortality, but the differences in stroke incidence was highly significant (P less than .001). The annual stroke rate, averaged over seven years, was 0.4% per year for group A, 1.47% per year for the surgically treated group, and 12.5% per year for groups B and C. The data indicate that group A ulcers have a benign prognosis, in noticeable contrast to group B and C ulcers which incur a high risk for subsequent stroke.