BACKGROUND Dietary supplementation with fish oil has been said to improve renal function in cyclosporin A (CsA)-treated subjects. METHODS Renal function and the acute renal haemodynamic and tubular response to an oral CsA-dose (3 mg/kg) were investigated before and after 12 weeks of fish oil supplementation (6 g/day) in 12 low-dose CsA-treated kidney-transplanted patients (s-creatinine, 124 +/- 24 mumol/l, mean +/- SD). After an overnight fast, ten l-h renal clearance periods were performed, two periods before and eight after CsA-ingestion. An additional control clearance study without CsA intake was performed in six subjects. RESULTS Fish oil did not change baseline values of the effective renal plasma flow (ERPF) or the glomerular filtration rate (GFR). Compared to the control study, CsA decreased GFR and ERPF significantly on average 20 +/- 3% (P < 0.01) and 23 +/- 3% (P < 0.01) respectively, 4-6 h after peak CsA blood concentration with no additional effect of fish oil. CsA also significantly decreased the renal clearance of lithium, used as an index of proximal tubular outflow, with no impact of dietary fish oil. CONCLUSIONS Fish oil supplementation had no effect on baseline renal function or CsA-induced hypoperfusion in stable renal transplant recipients treated with a low maintenance dose of CsA.