PURPOSE: The purpose of this article is to study whether the Doppler ultrasound differentiate obstructive hydronephrosis from nonobstructive hydronephrosis in sonographically diagnosed hydronephrosis. MATERIALS AND METHODS: We studied 35 kidneys of 27 patients with hydronephrosis which were diagnosed with sonography. Resistive indices were calculated from Doppler waveforms at the level of interlobar or arcuate arteries. We divided the materials into two groups with obstructive and nonobstructive hydronephrosis according to the findings of intravenous pyelography, retrograde pyelography, and anterograde pyelography. These were compared with normal control group(20 kidneys in 10 persons). We also grouped patients regarding to the duration after onset of clinical symptom, and graded the hydronephrosis into three group according to sonographic findings. We analyzed the correlations of the above sonography and clinical findings with resistive index. Hydronephrosis was obstructive in 20 kidneys(in 17 patients) and nonobstructive in 15 kidneys(in 10 patients). RESULTS: There was statistically significant difference between the mean resistive indices in obstructive hydronephrosis(0.74 ± 0.02) and that in nonobstructive hydronephrosis (0.60 ± 0.02)(p<0.05). Also there was no difference between the mean resistive indeces in nonobstructive hydronephrosis and that in normal control group(0.59 ± 0.07)(P > or = 0.05). CONCLUSION: The use of Doppler sonography can be helpful in detecting the presence or absence of true obstruction in patients with hydronephrosis.