We sought to evaluate whether results of Doppler ultrasound alone or in combination with morphologic sonographic evaluation could predict acute renal transplant rejection accurately enough to avoid biopsy. Seventy-one patients were evaluated at time of biopsy. A resistive index greater than 0.90 was 100% predictive of acute rejection, but sensitivity was only 26%. Resistive indices in acute and chronic rejection were often similar. When all morphologic features were abnormal (11%), acute rejection was uniformly present. Regression analysis selected resistive index over morphologic features for ability to predict acute rejection. When resistive index is greater than 0.90 or all morphologic features are abnormal, acute rejection can be predicted with high accuracy, but biopsy remains necessary in many cases.