Acute rejection is the most frequent cause of early graft failure. There is unanimity that Doppler sonography is a helpful method for the detection of complications after kidney transplantation. In the past, the indication for renal biopsy relied mainly on clinical assessment, although this assessment has not been standardised. Therefore, we conducted this prospective study to compare the value of sequential Doppler measurements with a standardised clinical rejection score, based on renal function, weight gain, graft swelling and tenderness. Fifty-eight patients (37 males, 21 females, mean age 46 +/- 12 years) after kidney transplantation were consecutively enrolled into the study. Doppler investigations were obtained within the first 24 h after transplantation, followed by an interval of 48-72 h. At the same time, a clinical examination was scored by a transplant physician blinded to the Doppler results. Clinical score and Doppler results, both were referred to the histological results of renal biopsy. In 24 out of 58 patients 25 acute rejections occurred. In seven patients, acute rejection was superimposed on primary graft failure. The cut-off levels for rejection were set at RI > or = 0.80 and PI > or = 1.70 based on receiver-operator curves using data from 663 Doppler examinations. Sensitivity and specificity was 72% for RI, and 72% and 74% for PI, respectively. The calculation of the intraindividual increase (deltaRI > or = 3%, deltaPI > or = 10%) did not improve these values. The clinical score revealed a sensitivity and specificity of 82% and 87%, respectively. The combined analysis of Doppler indices and clinical score showed a sensitivity of 96% with a specificity of 66%. Careful clinical monitoring alone using a clinical score is an appropriate procedure with which to decide about renal biopsy. Our data show that Doppler sonography should be performed within the first 24 h after transplantation to evaluate graft perfusion and baseline values. Afterwards, it should be used when clinical signs of rejection occur to underline the decision for renal biopsy even in borderline cases.