Acute veno-occlusive disease of the liver developed in seven of 29 patients undergoing bone marrow transplantation for treatment of leukemia, aplastic anemia, or disseminated carcinomatosis. All seven died despite successful marrow engraftment. Hepatic failure was the principal cause of death in four and contributory in three. The veno-occlusive disease did not relate to the nature of the pretransplant immunosuppressive regimen, since it occurred in patients receiving irradiation alone, chemotherapy alone, or both. Twenty-two of the patients were autopsied. Among these, the lesion was found in seven of 11 in whom a graft-versus-host reaction developed but in none of the 11 without such a reaction who had received similar pretransplant immunosuppression (P < 0.025). Hence, acute veno-occlusivedisease of the liver appears to be a complication of allogeneic bone marrow transplantation related to the development of a graft-versus-host reaction.