Objective : The purpose of this study was to determine the diagnostic accuracy of renal resistive index of base line, Δ RI ( difference between both sides) in diagnosis of acute obstruction.Patients and methods: 50 patients with suspected renal colic were evaluated by ultrasongraphy, IVU, renal duplex with determination of resistive index (RI) and the difference between RI of both kidneys (Δ RI). RI, Δ RI were considered positive with values of ≥ 0.70 and ≥ 0.05 respectively. IVU were considered the gold standard with which renal duplex were compared. Results : -Ultrsonography were normal in 10cases (20 renal units ) and show unilateral dilatation in 40 cases (40 renal units) so 40 unit showed dilatation and 60 unit normal .-IVU done for 40 patients with obstructed renal units .-Renal duplex showed that the mean (SD) RI was 0.74 (0.05) in those 40 dilated kidneys which is significant higher than the mean RI of 0.64 (0.05) in 60 normal kidneys.-The mean ΔRI of patients with acute unilateral obstruction was 0.09 (0.05) which was significantly higher than the mean Δ RI of patients with normal both kidneys 0.01(0.02).-RI was sensitive in 88% and specific 91%.-Δ RI was sensitive in 92% and specific 100% so had better diagnostic accuracy.-Obstructed kidneys showing delayed pelvicalyceal filling with contrast media (n= 27) had significantly higher values of RI and ΔRI than those obstructed kidneys without delayed filling (n=13).Conclusion : Renal duplex is a sensitive and highly specific test for the diagnosis of acute unilateral renal obstruction. It can replace IVU particularly in situations where IVU is undesirable.