Using Doppler ultrasonography, we evaluated the frequency and severity of carotid artery stenosis in 261 patients with carotid ischemic strokes, 813 patients with carotid transient ischemic attacks, 500 patients with asymptomatic neck bruits, and 500 controls. Most patients with strokes and transient ischemic attacks had no associated carotid artery disease (55% and 64%, respectively), and such patients without neck bruits were even more likely to be without carotid artery disease (69% and 77%, respectively). Carotid stenosis was more frequent and more likely to be severe in symptomatic than in asymptomatic patients (p less than 0.0002), even after adjusting for age and sex. Carotid stenosis is present in only a minority of patients with strokes and transient ischemic attacks, especially if neck bruits are absent, and the cause of the ischemic cerebral events in most of these patients remains unexplained.