This study aimed to determine whether dietary supplementation with fish oil has a beneficial effect on graft function and the incidence of rejection in renal allograft recipients treated with cyclosporin A (CsA). Renal function, blood pressure, the incidence of acute rejection episodes, graft survival, and renal histology and immunochemistry were investigated. In a randomized, placebo-controlled, double-blind trial, groups of 25 recipients of primary cadaveric renal allografts who had been treated with CsA took fish oil (30% C20:5 omega-3 and 20% C22:6 omega-3) or coconut oil (63% C8:0 and 36% C10:0) at 6 g/day for 3 months. There were no differences between the two patient groups with regard to HLA matching, panel-reactive antibody titers, or the demographic characteristics of donors or recipients. The GFR and effective RPF were determined at 1, 3, and 12 months after transplantation by simultaneous measurement of (125I-)iothalamate and (131I-)hippuran clearances. At 1 yr after transplantation, patients treated with fish oil showed better renal function than did the control patients, but this difference was not statistically significant. Blood pressure and antihypertensive drug use were similar in both groups. The number of rejection episodes was also similar, and renal histopathological and immunohistochemical studies showed no significant differences between the fish-oil group and the control patients. It is concluded that fish oil, at a dose of 6 g/day, has no beneficial effect after renal transplantation within the time scale of the study.