In this study, we examined the influence of pre-existing obesity (weight more than 120 per cent of ideal body weight) on outcome after renal transplantation. Among 263 cyclosporine-treated recipients of renal allografts, 223 (85 per cent) were nonobese and 40 (15 per cent) were obese prior to transplantation. Obese and nonobese recipients were similar with regard to age, sex, renal diagnosis, history of prior transplant, donor source, pretransplant blood pressure, pretransplant antihypertensive agents, diabetic recipients with insulin requirements more than 40 units per day and pretransplant serum cholesterol. Duration of hospitalization was similar (26 +/- 25 versus 25 +/- 14 days; p = NS). There was a significantly higher incidence of wound infections in obese recipients (17.5 versus 6.3 per cent; p = 0.036); other complications occurred with similar incidence. Preoperative per cent of ideal body weight correlated with post-transplant weight gain during the first post-transplant year (p = 0.00002). After one year, obese recipients had gained 14.2 +/- 2.2 kilograms compared with 8.9 +/- 0.6 kilograms for nonobese patients (p = 0.002). Mean doses of prednisone, azathioprine and cyclosporine were similar at three, six and 12 months post-transplant. There were no differences in blood cyclosporine, serum cholesterol or blood glucose levels at any time. Blood pressure measurements were similar throughout the first post-transplant year. There was no difference in the incidence or number of rejection episodes. Actuarial patient survival rate for nonobese patients was 93 per cent at three years. For obese patients, three year actuarial patient survival rate was 90.5 per cent (p = NS). Actuarial graft survival rate among nonobese patients was 71.8 per cent at three years. For obese patients, three year actuarial graft survival rate was 64.5 per cent (p = NS). In conclusion, obese patients with end stage renal disease are good candidates for renal transplantation. Obesity does not seem to constitute a major risk factor, the most notable adverse outcomes being an increased incidence of wound infection and continued weight gain through the first post-transplant year.