Calcineurin inhibitors have been used extensively in renal transplantation. Although highly effective, their nephrotoxicity may lead to graft dysfunction. Sirolimus is a nonnephrotoxic, potent immunosuppressive agent that blocks the G1 to S cell cycle by inhibiting signal transduction via a specific target protein. Sirolimus is useful in the initial immunosuppressive protocol or, as a conversion therapy, to treat chronic allograft nephropathy, calcineurin-inhibitor–related nephrotoxicity, and steroid-resistant rejection episodes after kidney transplantation. This study included 11 renal transplantation patients (7 men and 4 women; mean age, 34.4 years) in whom drug conversion had been implemented. We switched from a calcineurin inhibitor to sirolimus owing to chronic allograft nephropathy in 4 patients and to calcineurin-inhibitor–related nephrotoxicity in 7 patients. In chronic allograft nephropathy, the immunosuppressive protocol consisted of sirolimus (blood trough levels between 12-15 ng/mL) and prednisolone (10 mg/day). In all other cases, low-dose calcineurin inhibitors combined with low-dose sirolimus and prednisolone were used. For the tacrolimus and sirolimus combination, the sum of both drugs’ blood trough levels was kept between 12-15 ng/mL. For the cyclosporine and sirolimus combination, sirolimus blood trough levels were maintained between 8-12 ng/mL, and cyclosporine blood trough levels were kept between 100-150 ng/mL. Mean follow-up was 8 months (range, 4-24 months). After conversion to sirolimus, mean serum creatinine levels fell from 3.3 mg/dL to 2.4 mg/dL. In the 9 patients in whom we had switched to sirolimus early in the postoperative period, wound complications were prominent. Four of 9 patients had perirenal fluid collection and 3 of them need to be drained. Two of them had extravasation from the ureter, which we treated with a nephrostomy catheter and a double-J catheter. Serum cholesterol and triglyceride levels rose to a mean value of 227 ± 45 mg/dL and 276 ± 48 mg/dL, respectively. We observed mild thrombocytopenia in 1 case, which recovered within 1 month. No infectious complications were encountered. Data from this study demonstrate that sirolimus conversion may decrease serum creatinine levels in patients with chronic allograft nephropathy and calcineurin-inhibitor–related nephrotoxicity. However, it may increase wound-related complications in the early postoperative period. If doses are administered at appropriate serum levels, sirolimus may be safely used even in combination with a calcineurin inhibitor.