OBJECTIVE To determine the role of duplex Doppler ultrasonography (DDU) in patients with acute unilateral renal obstruction. METHODS A total of 161 patients with suspected renal colic due to urolithiasis were evaluated by DDU followed by intravenous urography (IVU). The mean intra-arterial resistive index (RI) and the difference of mean resistive index between both kidneys (delta RI) were determined for each person. An RI value of ≥0.70 and a delta RI value of ≥0.06 were taken as the discriminatory threshold for obstruction. IVU results were considered the " reference standard" against which renal DDU findings were compared. RESULTS IVU showed both kidneys to be normal in 51 patients and with unilateral ureteric obstruction in 110 patients. The mean RI for obstructed kidneys was 0.67 (0.048), which was significantly higher (P-value <0.05) than a mean RI of contralateral normal kidneys 0.59 (0.04). The mean delta RI in patients with unilateral ureteric obstruction was significantly higher than that in patients with normal kidneys, at 0.076 (0.03) and 0.03 (0.05), respectively. In patients with complete obstruction, sensitivity of RI and delta RI were 77.5% and 92.5% with a specificity of 84.3% and 90.1%, respectively. In patients with partial obstruction, the sensitivity of these values was 22.8% and 62.8% with a specificity of 84.3% and 90.1%. CONCLUSIONS Delta RI is more sensitive and specific than RI in acute renal obstruction. However, due to relatively low sensitivity for detection of partial obstruction, DDU cannot replace IVU as the standard imaging technique.