We investigated the velocity of renal blood flow on hydronephrosis using the pulse color Doppler ultrasound technique, comparing velocity on the healthy side with the diseased side of the renal, segmental and interlobar arteries. The velocities of the diseased side were markedly decreased, depending on the degree of hydronephrosis. These results suggest that the evaluation of renal arterial blood flow velocity in hydronephrosis is a useful examination for determining grade and predicting prognosis.