OBJECTIVE We assessed the value of renal duplex Doppler sonography in diagnosing obstruction in patients with acute renal colic. METHODS We compared the results of renal duplex Doppler sonography with those of intravenous urography (IVU) in 65 patients presenting with symptoms of renal colic. The Doppler spectral waveforms were used to calculate the resistance index (RI). The difference between the mean RIs (delta RI) in the 2 kidneys was calculated for each patient. The sensitivity, specificity, and accuracy of delta RI for the diagnosis of obstruction were calculated with several discriminatory thresholds. RESULTS All patients had an obstruction on IVU. When a high delta RI threshold for obstruction (delta RI > or = 0.1) was applied, the sensitivity and specificity of duplex Doppler sonography were 10% and 80%, respectively. When the delta RI threshold was decreased, the sensitivity increased but the specificity decreased, and both remained quite low. Twelve patients had a lower mean RI in the obstructed kidney than in the normal contralateral kidney. Among 14 patients without pyelocaliceal dilatation, the sensitivity of duplex Doppler sonography was 70% with a delta RI threshold of 0.03. CONCLUSIONS Duplex Doppler sonography is not sensitive enough to diagnose obstruction in patients with acute renal colic.