The effect of a daily supplementation of 6 g fish oil (30% C 20 : 5 omega-3 = EPA and 20% C 22 : 6 omega-3 = DHA) for 1 month on renal function variables was investigated in a placebo-controlled (6 g coconut oil), prospective, randomized, double-blind study in acute postoperative cyclosporin A (CyA)-treated renal transplant recipients. Seventeen patients ingested placebo capsules ( E P A -) and 14 patients fish oil (EPA + ). Renal function tests were performed using the simultaneous determination of 125I-iothalamate and 131I-hippuran clearances for glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), respectively. Renal reserve filtration capacity was assessed by dopamine infusion, amino acid infusion, and a combination of both stimuli. After i month there were no significant differences in rejection episodes, CyA dose, or CyA levels. In contrast to our earlier observations, serum creatinine, creatinine clearance, GFR, and ERPF did not differ between the EPA -and EPA + groups. Filtration fraction (FF) differed significantly, being 0.21 in the EPA -group versus 0.26 in the EPA + group. To exclude the possible influence of a rejection episode, the nonrejecting patients were analyzed separately, creating the subgroups EPA + re -and EPA -re -. These two groups were comparable in age, donor age, and GFR. The EPA + r egroup had a significantly lower ERPF (164 ml/min per 1.73 m 2) than the E P A -r e -group (262 ml/min per 1.73 m2). FF was significantly higher in the EPA + regroup (0.26) than in the EPA -re -group (0.21). Following dopamine, no significant differences in the percentage increase of GFR and ERPF between both groups were observed, while FF fell to the same extent in both groups. Following amino acids, the fish oil-treated patients had a significantly better response on GFR (EPA + r e -15.3 versus EPA -re -10.6%; P < 0.05). The near-normal FF and the better response on amino acid infusion strongly suggest that at i month postoperatively, the CyA-and fish oil-treated patients have more balanced renal hemodynamics than the CyA-and coconut oil-treated patients.