Tacrolimus is a calcineurin inhibitor immunosuppressant that has been widely used in the last decade. The recommended doses and therapeutic ranges have changed from the beginning because now new combinations of immunosuppressants are used and the tendency is to maintain low tacrolimus levels in order to avoid undesirable side effects. As there is a great pharmacokinetic variability between patients we have a pediatric liver transplant protocol to calculate the area under the curve (AUC) in order to avoid over-immunosuppression in patients with risk of lymphoproliferative disorder. We have studied the influence of hepatitis C virus infection (HCV) on tacrolimus requirements in patients with kidney transplant, and we observed that HCV+ patients need lower tacrolimus doses to maintain the same level as HCV-patients. These studies clearly contribute to improving drug effectiveness and thus to achieving an adequate immunosuppressive therapy.