Latent herpes viruses such as herpes simplex virus, cytomegalovirus (CMV), and varicella zoster virus are often reactivated after bone marrow transplantation, giving rise to infections. In contrast, Epstein-Barr virus infections rarely occur. Significant mortality is induced especially by pneumonitis, most often caused by CMV. Immunosuppression and pancytopenia caused by CMV increase the risk of bacterial infections and invasive fungal infections. Herpes viruses may increase the risk of acute and chronic graft-versus-host disease (GVHD). Thus, immunity to several herpes viruses was associated with an increased risk of acute GVHD. Seropositivity for CMV in recipient and donor increased the risk of chronic GVHD. Herpes viruses were also associated with a decreased risk of leukemic relapse. CMV infection, asymptomatic CMV infection, and seropositivity for several herpes viruses were associated with a reduced incidence of relapse in different reports. In spite of this possible antileukemic effect, leukemia-free survival was unaffected by herpes virus immunity in recipients or donors.