Duplex Doppler sonography recently has been used to evaluate renal transplants. Some authors have stated that high resistive indexes (RIs) occur in the presence of acute renal transplant rejection. RIs less than 0.7 are considered as probably excluding acute transplant rejection. We performed a prospective study of duplex sonographic examinations of pediatric patients (mean age, 8 years; 13 boys, two girls) with renal allografts and clinically suspected transplant disease. The results of 22 duplex studies were correlated with histopathologic data obtained between July 1987 and June 1988. RIs of the arcuate arteries in patients with acute rejection (n = 14) averaged 0.62 (range, 0.50-0.80). The RI in patients with chronic rejection (n = 1) was 0.59. RIs in patients with acute tubular necrosis (n = 3) averaged 0.66 (range, 0.59-0.72). RIs in patients with cyclosporine A toxicity (n = 4) averaged 0.66 (range, 0.58-0.79). Tubulointerstitial rejection was predominant, with only two patients showing minimal acute vascular rejection. Thirteen of 14 pediatric patients with histologically proved renal transplant rejection had a resistive index of less than 0.70. This study refutes the concept that resistive indexes of less than 0.7 exclude acute rejection.