Duplex ultrasound scan (DUS) criteria for grading >50% carotid artery stenosis is typically divided into broad categories such as 50-79% stenosis, 80-99% stenosis, and occlusion. The purpose of this study is to validate DUS criteria for stratifying 50 to 100% carotid stenosis into 10% intervals using digital substraction cerebral angiography (DSCA) as the standard of comparison. Between 1996 and 2001, 163 patients were evaluated with duplex ultrasound and angiography. A total of 326 carotid arteries were studied using DUS in an accredited ICAVL vascular laboratory. Threshold velocity criteria for determining the degree of carotid stenosis was defined according to seven categories: <50%, 50-59%, 60-69%, 70-79%, 80-89%, 90-99%, and occlusion. Treatment decisions were based on the angiographic findings. In cases where the degree of stenosis as defined by duplex velocity criteria did not correlate with angiographically defined stenosis, each record was reviewed to determine whether the angiographic findings altered the surgeon's treatment decision. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for DUS-defined degree of stenosis as compared to angiographically defined stenosis were determined. There was a high correlation (R = 0.96) between duplex scan and angiography in 93% (302/326) of the cases. Clinical management was altered in only 3% (10/326) of the cases because of the results of angiography. The DUS velocity criteria to grade the severity of carotid disease in 10% intervals is reliable and accurate. Clinical management of patients with carotid stenosis can be based solely on carotid DUS in 97% of patients considered for treatment of carotid artery disease.