Allogeneic bone marrow transplantation (BMT) has been shown to result in long-term disease-free survival in patients with leukemia. However, the utility of this treatment approach is limited by treatment-related morbidity and mortality. We present an update of a study in which a BMT preparative regimen consisting of a 4-day course of busulfan and a 2-day course of cyclophosphamide (BuCy2) was used in patients with chronic myelogenous leukemia. Patient survival depended on disease stage, with a 58% survival rate for patients in first chronic phase, 41% for those in accelerated phase, and 25% for those in the blast transformation stage. There was a significant difference in patient survival between those who received transplants within 1 year of diagnosis and those who received transplants more than 1 year after diagnosis (70% v 40%). This difference appeared to be due to the extent of previous exposure to busulfan. The overall mortality rate in this study was 46%. We conclude that the BuCy2 preparative regimen is similar in effectiveness to regimens that include total body irradiation and results in comparable levels of transplant-related mortality. Our results strongly indicate that patients with chronic myelogenous leukemia who receive BMT within the first year after diagnosis have a significantly better clinical outcome than those who receive BMT later in the course of the disease.