In a single-center prospective randomized controlled study, the impact of calcineurin inhibitor (CNI) reduction or withdrawal on the pharmacokinetics of mycophenolic acid (MPA) was studied in a group of renal transplant recipients with impaired renal function. Mycophenolate mofetil (MMF) was added to a baseline regimen of prednisolone and CNI. Afterwards the patients were randomized into "CNI withdrawal" and "CNI continuation" groups. The dosage of CNIs, cyclosporine or tacrolimus, was gradually reduced and withdrawn within 6 weeks from patients in the withdrawal group. The continuation group was maintained on therapy with CNI, MMF, and steroids. These regimens were maintained until the ninth month. In contrast to the withdrawal of tacrolimus, which has no significant effect on MPA pharmacokinetics, cyclosporine withdrawal was associated with a significant increase in the trough levels and areas under the curve of MPA. Serum creatinine and urine albumine levels stabilized on average after CNI withdrawal in this population. The results are consistent with the hypothesis that cyclosporine attenuates the enterohepatic recirculation of MPA. The withdrawal of CNI has a positive effect on renal function in chronic allograft dysfunction.