Thirty-five children who underwent kidney transplantation were prospectively investigated with doppler ultrasound on day 3, 10 and 30 posttransplant, and at the time of acute dysfunction episodes. Resistive index was obtained from graft artery and was significantly elevated in patients with acute rejection episode or acute tubular necrosis (P < 0,05) without any significant difference between these two conditions. Children with episodes of cyclosporine nephrotoxicity displayed the same signal as children with normal graft function. Even if a daily follow-up shoul improve the specificity and the sensibility of doppler ultrasound examination, its ability to differenciate acute tubular necrosis from acute allograft rejection is not available enough. Further studies are required to evaluate duplex colour doppler ultrasonogranphy in such conditions.