Color flow and pulsed Doppler ultrasound measurements of renal artery blood flow were compared with cardiac output in 22 preterm and 19 full-term healthy neonates. Renal arteries were insonated 3-5 mm from the abdominal aorta at an angle of less than 15 degrees. Vessel diameter was estimated from color flow diameter. Total renal blood flow increased significantly (P less than .001) with advancing birth weight and gestational age, because of increased vessel diameter, but flow velocity did not. The mean (+/- 1 standard deviation) value for both right and left renal artery blood flow was 21 mL/min/kg +/- 5, and the mean proportion of cardiac output to the kidneys was 16.1% +/- 3.7. Seven preterm infants with a symptomatic patent ductus arteriosus had increased cardiac output values (444 mL/min/kg +/- 45) and reduced right (15 mL/min/kg +/- 7) and left (16 mL/min/kg +/- 7) renal artery blood flow, with 6.6% +/- 2.0 of cardiac output directed to the kidneys. These values normalized after closure of the ductus. This study provides normative data for renal artery blood flow in neonates during the first week of life.