Left coronary artery flow velocities in neonates were determined noninvasively with transthoracic pulsed wave Doppler ultrasonography. In normal subjects (n = 30) there was diastolic flow predominance with median (range) peak flow velocity in diastole, 23.8 cm/s (12.7 to 51.3 cm/s), and median peak flow velocity in systole, 12.7 cm/s (7.8 to 35.0 cm/s). In 3 neonates with severe aortic stenosis, retrograde left coronary flow throughout systole was observed before surgery. In these patients there was forward systolic flow 4 to 8 days after successful surgical valvulotomy was performed.