We retrospectively studied the results of duplex scanning for evaluation of renal artery disease in 158 patients. Satisfactory examinations were achieved in 144 patients (90%). Arteriograms were available for 43 renal arteries. We used the ratio of the peak velocities in the renal artery and the aorta (RAR) to separate nonstenotic arteries (less than 60% diameter reduction) from stenotic arteries (greater than 60% diameter reduction). With an RAR of greater than 3.5 to indicate stenotic lesions, duplex scanning had a sensitivity of 91% (20 of 22 diseased arteries correctly identified) and specificity of 95% (20 of 21 normal or insignificantly diseased arteries correctly identified). One of four occluded arteries was incorrectly interpreted as patent because of misidentification of a collateral vessel. Prospective studies will be necessary to validate this test and establish other criteria for a more detailed classification of renal artery stenosis. The ratio of the end-diastolic to peak systolic velocities in the renal artery (EDR) tended to decrease with increasing serum creatinine levels, presumably because renal vascular resistance increases with end-stage parenchymal disease. EDR may prove useful in the detection of advanced parenchymal disease before renal artery revascularization is attempted.