Use of allogeneic bone marrow transplants continues to increase. During the 36-year period between 1955 and 1990, more than 33,000 patients received allogeneic bone marrow transplants; more than 45% of these were performed during the 3 years 1988-1990. Transplants are effective therapy for leukemia and other hematologic diseases. It is widely considered that transplants are the treatment of choice for aplastic anemia and chronic myelogenous leukemia, those who fail conventional therapy for acute leukemia and a variety of genetic, metabolic and immune deficiency disorders. Successful application of bone marrow transplantation is limited by complications such as graft failure, graft versus host disease GVHD and interstitial pneumonia and, until recently, the requirement for an HLA-identical sibling donor. In the past few years, an increasing number of transplants were performed using unrelated or HLA-partially matched related donors with some success. Development of post-transplant complications can often be predicted by risk factor assessment. In this report, current data from the IBMTR are summarized and several risk factors affecting outcome identified.