Utilization of bone marrow transplantation as a therapeutic modality continues to increase. More and more institutions are initiating bone marrow transplant programs. During the 33-year period between 1955 and 1987, more than 20,000 patients received allogeneic bone marrow transplants; more than 50% of these were performed in the 3 years, 1985-1987. Transplantation is an effective therapy for acute leukemia; in some instances it is the preferred treatment. In chronic myelogenous leukemia, severe aplastic anemia, and some genetic and immune deficiency diseases, bone marrow transplantation provides the only possibility for cure. Bone marrow transplantation is associated with serious problems such as graft-versus-host disease (GvHD), graft failure, interstitial pneumonitis and, until recently, the requirement for an HLA-identical sibling donor. In the past few years, an increasing number of transplants have been performed using HLA-partially matched related or unrelated donors with some success, the level of which is yet to be determined. The development of acute GvHD (8) and interstitial pneumonitis (9, 10) can often be predicted by risk factor assessment. Special precautions can then be taken for patients at high risk of these complications. In this report, current data from the International Bone Marrow Transplant Registry were summarized and several risk factors affecting outcome were identified.