Liver transplant patients receiving tacrolimus, and who experience side effects such as hypertension and renal dysfunction, will be converted to sirolimus with low-dose tacrolimus, or Tacrolimus withdrawal. This study will evaluate allograft function by serial clinical lab testing, the pharmacokinetics of sirolimus and tacrolimus, the glomerular filtration rate (GFR) and the potential side effect of sirolimus, such as marrow suppression and hyperlipidemia. Two pharmacokinetic evaluations are planned: once around the third post-transplant month and another one at about 12 months. Expected outcomes are, a better understanding of sirolimus pharmacokinetic parameters over time in pediatric/adult liver recipients and early efficacy and safety data of the sirolimus as a non-nephrotoxic alternative to tacrolimus.