In renal graft recipients primarily treated with cyclosporin and low-dose methylprednisolone, withdrawal of the long-term steroid medication increases the likelihood of developing rejection episodes. In order to determine the predictive value of clinical parameters and routine prewithdrawal graft biopsies for the risk of rejection, the authors studied 141 kidney recipients from whom steroids were withdrawn 7-9 months after transplantation in a clinically stable situation. Both the quality of the HLA-match and the results of prospective graft biopsies were found to correlate significantly to the occurrence of acute rejection. In order to investigate the influence of additional azathioprine medication on the incidence of acute rejections in recipients not receiving steroids, immunosuppression was continued with cyclosporin monotherapy in 88 patients and with cyclosporin plus azathioprine in 53 patients. The risk of developing rejection episodes was significantly reduced from 48% after 1 year on monotherapy to 28% after the addition of azathioprine medication.