Background: Previous studies indicate that obesity is a risk factor in renal transplantation. However, these analyses did not control for variable donor factors that may strongly influence outcome. To control for donor variables such as age, cause of death, procurement techniques, preservation methods, cold ischaemia time and implantation technique, we analysed patient and graft survival in recipients of paired kidneys, derived from the same procurement procedure, preserved in the same manner, subjected to similar cold ischaemia time and implanted by the same surgical team. Between June 1992 and August 1999, 28 procurement procedures provided kidneys which were transplanted into one obese and one non-obese recipient. Body mass index (BMI) was calculated as kg/m2. Recipients were classified as obese (BMI > 30) or non-obese (BMI < 30). Immunotherapy for all recipients consisted of a triple therapy regimen of cyclosporine or prograf, azathioprine or cellcept, and prednisone. Patients with delayed graft function (DGF), defined as the need for dialysis within 72 h of the transplant procedure, were treated with anti-thymocyte globulin (ATG) or thymoglobulin (TMG) induction for 5–7 d. The rate of DGF (7.1 versus 10.7%) and acute rejection (39.3 versus 35.7%) were similar in the obese and non-obese recipient groups. Patient survival was similar at 1, 3 and 5 yr in both groups. In addition, graft survival was similar at 1 yr. However, a trend toward decreased medium-term graft survival, which reached significance at 5 yr, was observed in the obese group. Furthermore, mean serum creatinine at 1 yr was higher in the obese group (2.0) compared with the non-obese group (1, 4) (p=0.12). This analysis of paired cadaver kidneys indicated that obesity is not a risk factor for DGF, acute rejection, and 1-yr graft survival. However, a decreased medium- and long-term graft survival trend, which reached statistical significance at 5 yr, was observed in obese recipients.