* Herpesvirus infections may be severe in immunocompromised patients, with a high risk of complications and mortality. * Recipients of hematopoietic stem cell transplant (HSCT) or patients receiving high-intensity chemotherapy for hematological malignancies are the most vulnerable individuals. * Although the worldwide prevalence of herpes simplex virus 1 (HSV-1) and varicella-zoster virus (VZV), antiviral prophylaxis in seropositive HSCT recipients has significantly reduced the rate of infection. * Acyclovir (ACV) is the first-choice drug for the prophylaxis or the therapy of that kind of infection. * Since the beginning, ACV has demonstrated to be characterized by a large interpatient variability, especially in children. * Therefore, therapeutic drug monitoring and pharmacokinetic studies may help in optimizing drug in children with malignancies.