Renal impairment is common in patients after liver transplantation and is attributable in large part to the use of calcineurin inhibitor (CNI)-based immunosuppression. We sought to determine whether conversion to sirolimus-based immunosuppression was associated with improved renal function. In a single-center, randomized, controlled trial, 30 patients at least 6 months post liver transplantation were randomized to remain on CNI-based immunosuppression or to switch to sirolimus-based immunosuppression. The primary outcome measure was change in measured glomerular filtration rate (GFR) between baseline and 12 months. Of 30 patients randomized, 3 were withdrawn at randomization, leaving 14 patients on CNI and 13 on sirolimus. There was a significant improvement in delta GFR following conversion to sirolimus at 3 months (7.7 mL/minute/1.73 m2; 95% confidence interval, 3.5-11.9) and 1 yr (6.1 mL/minute/1.73 m2; 95% confidence interval, 0.9-11.4). The difference in absolute GFR between the 2 study groups was significant at 3 months (P = 0.02), but not at 12 months (P = 0.07). The principal adverse events following conversion were the development of skin rash (9 of 13 patients, 69%) and mouth ulcers (5 of 13 patients, 38%). Two patients developed acute rejection at 2 and 3 months following conversion, 1 in association with low sirolimus levels and 1 having stopped the drug inadvertently. In conclusion, overall, this study suggests that conversion to sirolimus immunosuppression is associated with a modest improvement in renal function. Side effects were common, but tolerable in most patients and controlled with dose reduction. Liver Transpl 13:1694–1702, 2007. © 2007 AASLD.