Seven hundred sixty-seven primary renal allografts from a single center were divided into subgroups according to combinations of several major risk factors: donor source and histocompatibility match, age, and presence or absence of diabetes. The relative effect of diabetes on patient and graft survival decreased as histocompatibility differences increased. The influence of recipient age, however, dramatically decreased as histocompatibility differences decreased. In all groups donor source and histocompatibility match had the strongest relative effect in determining subsequent 2-year patient and graft survival.