Given the strong association found in this study between CYP3A5*1/*3 and CYP3A5*1/*1 with Tac pharmacokinetics in the immediate post-transplant period and acute GVHD incidence, these results support the adoption of preemptive pharmacogenetic testing as a tool to better predict each patient’s Tac initial dose, helping to achieve the therapeutic range and reduce the risk of acute GVHD earlier. This evidence concerns the gene CYP3A5 and acute graft versus host disease.