Blood flow velocities and pulsatory indices in both renal arteries (RAs) and in the internal carotid artery (CAI) were measured by pulsed Doppler ultrasonography in ten preterm infants with patent ductus arteriosus (PDA), before and after surgical ligation. The results obtained in the RAs were compared to those found in a reference group of 22 stable preterm infants. In the RAs the diastolic steal volume of the PDA led to a marked decrease in diastolic blood flow velocity (range 3 to -23 cm/s). Seven infants showed retrograde diastolic flow, whereas only three infants had these flow patterns in the CAI. In the RAs, the peak systolic blood flow velocities (range 56 to 135 cm/s) exceeded the values found in the reference group by 85% on average. The pulsatility indices reached values of above 1,00. In spite of the increase in systolic flow velocities before surgery, the time mean of maximum velocities was significantly lower than those measured after surgery and in the reference group. After PDA ligation, blood flow velocities normalized. The present study shows that a large PDA may induce abnormal flow patterns even in the RAs. These flow patterns may predispose to renal hypoperfusion and subsequent impairment of renal function.