Serial Doppler ultrasound examinations of the hepatic veins were performed on 50 consecutive paediatric liver transplants. Damping of the normally pulsatile signal was observed in 23 of the 32 biopsy-proven episodes of rejection. In 10 episodes, the reduction in hepatic vein pulsatility preceded clinical and biochemical evidence of rejection by up to 36 h. Seven cases had damped signals throughout the post-operative period which precluded assessment by this method. In two patients the hepatic vein signals remained pulsatile despite rejection, one patient having unsuspected tricuspid regurgitation, and the other a stenotic IVC anastomosis. In the 35 liver transplants with normal pulsatility, hepatic vein Doppler proved to be a valuable indicator of acute rejection during the first 2 weeks following transplantation (sensitivity 92%, specificity 100%, positive predictive value 100% and negative predictive value 83%).