A series of 125 renal transplants were analyzed in order to ascertain the characteristics of accelerated allograft rejection. An intense accelerated rejection could be identified within the first five days in 12 of 67 transplants (18%) with good immediate renal function. Accelerated rejection differed from the usual acute rejection reaction by higher fever, increased duration and intensity of the rejection, and increased difficulty in reversing the reaction. Accelerated rejection was reversible and associated with satisfactory renal function one year posttransplant in 58% of patients. Lymphocytotoxic and heterophile antibodies in preoperative serum and eluates of removed kidneys were not present. Contrary to recent reports, accelerated renal allograft rejection is a potentially reversible process and not necessarily due to humoral antibody presensitization.