Seventy renal allograft biopsies were done in 31 patients, routinely at 1, 2, and 3 months posttransplant, and as clinically indicated, using an automated biopsy "gun." The histological diagnosis was made according to the Banff schema, which emphasizes tubulitis and vascular inflammation over mononuclear cell infiltration. Fifty-three biopsies satisfied histological inclusion criteria. Twenty-nine biopsies were obtained from stable patients, defined as those in whom serum creatinine had changed < 10% in 2 weeks, and in whom immunosuppression (cyclosporine, azathioprine, and prednisone) had not been increased in that interval. Of these biopsies, 30% (9/29) showed rejection, which could not have been predicted from pretransplant (HLA mismatch, panel-reactive antibody titer) or posttransplant (cyclosporine and serum interleukin 2 receptor levels) variables. The significance of these early subclinical rejection episodes is unknown, and their effects on long-term graft histology and function are being examined in a controlled study.