Optimal criteria for the duplex sonographic diagnosis of carotid artery stenosis have not yet been defined. We studied 205 vessels in 105 patients with both duplex sonography and angiography. Four diagnostic groups were defined on the basis of Doppler flow characteristics. Receiver-operating-characteristic curves were used to compare diagnostic criteria at significant stenosis levels, and to select threshold values that emphasize specificity as well as sensitivity. Peak systolic velocity, systolic velocity ratios, and end-diastolic velocity were all shown to be equivalent predictors of significant disease. We chose peak systolic velocity in the internal carotid artery as our best parameter because of its ease of measurement. The use of combined parameters offered no significant statistical advantage over the use of a single parameter. The real-time assessment of stenosis and ulceration was not found to be reliable. ROC curves should be used to select Doppler criteria with desired sensitivities and specificities to maximize the benefit in each clinical setting.