Reliable differentiation between obstructive and non-obstructive hydronephrosis is decisive for the further therapeutic management in infants. The results of renal Doppler sonography were compared with diuretic radionuclide renography and with the follow-up results in 33 patients (range: 21 to 98 days). In Doppler sonography, a resistive index (RI) of > 0.9 was considered to be abnormal in the sense of an obstruction. In diuretic renography, a T1/2 value (time until a 50% decrease in activity in the kidneys was observed after injection of furosemide) of >20 min was appraised as obstructive hydronephrosis. In six patients an obstructive (T1/2 >20 min) and in 27 patients a non-obstructive (T1/2 <20 min) hydronephrosis was found. All patients with obstruction in diuretic renography showed an abnormal RI (>0.9) in Doppler sonography. In addition, all patients with surgery and obstruction in diuretic radionuclide renography showed an improvement in hydronephrosis. However, seven patients had a false-positive result in Doppler sonography. All patients with non-obstruction in diuretic radionuclide renography showed no worsening of hydronephrosis without surgery in the follow-up. We found a RI of 0.84 +/- 0.07 in the non-obstructive group and of 0.96 +/- 0.05 (p = 0.018) in the obstructive group. Doppler sonography showed discrepant results compared to diuretic radionuclide renography and therefore cannot replace this method.