Traditionally, chronic calcineurin inhibitor (CNI) nephrotoxicity has been considered to be one of the main nonimmune mechanisms causing chronic renal allograft dysfunction. CNI minimization and withdrawal strategies have yielded inconsistent results. Few studies address the feasibility of CNI elimination in a prednisone-free regimen. We report a prospective, randomized trial in 200 patients evaluating the impact on renal function and incidence of acute rejection after conversion from tacrolimus (Tac) to sirolimus (SRL). Patients with recent (<3 months) acute rejection episodes or with >0.5 g/day of proteinuria were excluded. All were induced with alemtuzumab, underwent rapid steroid elimination and were maintained on mycophenolate mofetil and Tac. At 12 months posttransplant, patients were randomized 2:1 to SRL (n = 123) or maintained on Tac (n = 64). Mean follow-up was 41.1 +/- 15.8 months in the SRL group and 40.7 +/- 14.4 months in the Tac group. Biopsy-proven acute rejection at 24 months postrandomization was similar between the groups. Patient survival, graft survival and estimated GFR were also not statistically different. Our study demonstrates that in a prednisone-free immunosuppressive regimen, conversion from Tac to SRL at 12 months posttransplantation is not associated with increased rates of acute rejection and graft loss. However, despite CNI elimination, renal allograft function is equally maintained in both groups.