BACKGROUND: The replacement of Sandimmun by Neoral in 1995 was thought to cause subsequent renal function deterioration due to the better bioavailability of the new drug. We prospectively analyzed the effect of a dose-to-dose drug replacement on renal function over 12 months. METHODS AND RESULTS: The renal function of 47 consecutive heart transplanted patients was prospectively evaluated before (T0), at 1 (T1), 3 (T3), and 12 (T12) months after drug replacement. Mean serum creatinine was not significantly different at T0 and T12 (142 +/- 55 and 154 +/- 60 micromol/L, p = 0.1). We were able to reduce cyclosporine total and weight-indexed doses by, respectively, 11% and 14% between T0 and T12 (274 +/- 86 to 244 +/- 72 mg/d, p = 0.0003; and 3.7 +/- 1.4 to 3.2 +/- 1.2 mg/kg/d, respectively, p = 0.0005). CONCLUSIONS: This study demonstrates that the dose-to-dose replacement of Sandimmun by Neoral is feasible, with no direct influence on renal function over a 1-yr follow-up.