AbstractMost renal transplants are lost by rejection. A method is required to identify and discriminate between this and acute tubular necrosis and cyclosporin toxicity. The sonogram of the normal renal transplant is characteristic. Early Doppler studies measuring a rise in vascular impedance in acute rejection showed a high sensitivity and specificity. This appears, at least in part, to have been due to patient selection. We conclude that Doppler studies cannot be used to differentiate between the main parenchymal causes of renal transplant failure, although it can be helpful, with other clinical information, in supporting a diagnosis and in monitoring the effects of treatment. © John Wiley & Sons, Inc.