Acute injury and age are characteristics of transplanted tissue that influence many aspects of the course of a renal allograft. The influence of donor tissue characteristics on outcomes can be analyzed by studying pairing, the extent to which two kidneys retrieved from the same cadaver donor manifest similar outcomes. Pairing studies help to define the relative role of donor-related factors (among pairs) versus non-donor factors (within pairs). This study analyzed graft survival for 220 pairs of cadaveric kidneys for the similarity of parameters reflecting function and rejection. It also examined whether the performance of one kidney was predicted by the course of its "mate," the other kidney from that donor. Parameters reflecting function showed sustained pairing posttransplantation, as did graft survival. In contrast, measures of rejection strongly affected survival but showed no pairing. Surprisingly, the survival of a kidney was predicted by the early performance of its mate, an observation we term the "mate effect." Six-month graft survival and renal function were reduced in grafts for which the mate kidney displayed any criteria for functional impairment (dialysis dependency, low urine output [</=1 L] in the first 24 h posttransplant or day-7 serum creatinine >/= 400 micro mol/L), even for kidneys which themselves lacked those criteria. Rejection measures did not demonstrate the mate effect. In conclusion, kidney transplant function is strongly linked to donor-related factors (age, brain death). In contrast, rejection affects survival and function, but it is not primarily determined by the characteristics of the donor tissue. Graft survival reflects both of these influences.