Infections occurring 6 mo or later after bone marrow transplantation for severe aplastic anemia or hematologic malignancy were analyzed in 98 long-term survivors. Varicella-zoster (VZ) infections were analyzed separately from all other infections. The factor predisposing most strongly to late VZ infection was genotypic nonidentity for HLA between marrow donor and recipient. There was a suggestion that chronic graft- versus-host disease (GVHD) associated with the presence of nonspecific suppressor cells also predisposed to late VZ infection, while age less than 10 yr was protective against such infections. Chronic GVHD predisposed to late non-VZ infections, but this was not increased by the presence of nonspecific suppressor cells. HLA nonidentify between patient and marrow donor further increased the risk of late non-VZ infections over and above that due to the presence of chronic GVHD. Receipt of a syngeneic transplant appeared protective for late non-VZ infections. These findings suggest that full genotypic identity for HLA between donor and recipient may be required for optimal immune reconstitution after marrow transplantation and may denote a possible biologic role for nonspecific suppressor T cells in humans.