Abstract: Calcineurin inhibitors have dramatically improved the outcomes of pediatric liver transplantation. However, calcineurin inhibitor use is associated with a 50% reduction in glomerular filtration rate in the first year post-transplant. Nephrotoxicity can be difficult to manage, especially in the pediatric population. We hypothesized that the addition of an mTOR inhibitor with decreased calcineurin inhibitor levels might improve or prevent renal insufficiency and improve control of rejection. A retrospective chart review was performed on the patients treated with sirolimus who had undergone an orthotopic liver transplant between January 2000 and February 2003. Thirty-eight patients were identified. Mean age was 8.6 yr. Fourteen patients were male and 24 were female. Mean weight was 30.3 kg. The most common indications for starting sirolimus were rejection (42%) and renal impairment (29%). Seventy-three percent of patients begun on sirolimus remain on the medication. Those with renal impairment (11 patients) showed improvement in their creatinine levels from a mean baseline of 1.3 to 0.8 mg/dL. Their calculated creatinine clearance (Schwartz formula) improved from 63.7 to 84.8 mL/min (p = 0.03). Patients started on sirolimus for rejection showed significant improvement in hepatocellular enzymes despite a reduction in the tacrolimus level from 12.2 to 7.5 ng/mL. The mean alanine aminotransferase level improved from 221 to 100 units/L (p = 0.02), and the mean aspartate aminotransferase improved from 121 to 99 units/L (p = 0.59). Addition of sirolimus to a tacrolimus-based regimen with lower target tacrolimus levels improved liver function in patients with rejection. Addition of sirolimus significantly improved renal function as shown by creatinine level and calculated creatinine clearance in those children with renal impairment. The effect of combined immunosuppressant treatment with tacrolimus and sirolimus on long-term renal function needs to be evaluated.