Renal transplantation is the treatment of choice of the chronic renal insufficiency arrived at its final stage. Tacrolimus is an immunosuppressant treatment used for the prevention of episodes of acute rejection. Tacrolimus is characterized by a narrow therapeutic index and important interindividual variations of its pharmacokinetic characteristics. Proteins CYP3A4 and CYP3A5 are responsible of intestinal and hepatic metabolism of Tacrolimus. Various polymorphisms for CYP3A5 and CYP3A4 were described and several retrospective studies suggested an association between a genetic polymorphism of CYP3A5 and the pharmacokinetic parameters of Tacrolimus. In particular, we showed that the presence of an allele CYP3A5*1 was associated to the use of more important amounts of Tacrolimus to obtain the desired blood concentrations.