Antithymocyte globulin (ATG) reduces chronic GVHD; however, effect of ATG on prognosis of allogeneic hematopoietic stem cell transplantation(HSCT)differs depending on characteristics of HSCT. In myeloablative conditioning(MAC) , ATG has been shown to inhibit chronic GVHD without increase in relapse or decrease in survival, whereas in reduce intensity conditioning, ATG was also shown to inhibit chronic GVHD as in MAC; however, effect of ATG on relapse or survival after HSCT is still unclear. Prophylactic use of ATG is recommended in peripheral blood stem cell transplantation(PBSCT)or HLA 1-locus mismatched HSCT, while not recommended in cord blood transplantation. Although an optimal dose of ATG remains undefined, lower doses of ATG has been used in recent studies. In a nationwide retrospective study for ATG in unrelated PBSCT, low dose ATG significantly reduced the incidence of chronic GVHD. Recently, an association between absolute lymphocyte counts(ALC)before the administration of ATG and transplant outcomes has been reported, suggesting a strategy to individualize ATG dosing according to ALC before ATG.