A retrospective analysis compared the sensitivity, specificity, and predictive value of two methods using noninvasive Doppler ultrasound for detecting the severity of carotid artery disease. Doppler spectral analyses were used in determining the peak systolic frequency (PF) found within the stenosis, as well as the ratio of the peak systolic frequency distal to the stenosis, to the peak systolic frequency within the stenosis (f2/f1). The results of the noninvasive tests were compared with carotid arteriography. The sensitivity of the two methods were similar, but the specificity of f2/f1 was much higher (98%) than PF (88%) with stenosis greater than 50%, and marginally better, 97 and 94%, respectively, with stenosis greater than 75%. The positive predictive value was also better for f2/f1 than PF; 91% and 71% for stenosis greater than 50%, and 77% and 67%, respectively for stenosis greater than 75%. The use of Doppler frequency ratio in conjunction with peak frequency measurements provide a quantitative and fairly accurate evaluation of the severity of carotid artery disease.