Bone marrow transplantation (BMT) preparative regimens consisting of busulfan (Bu) and cyclophosphamide (Cy) were developed as an alternative to preparative regimens based on total body irradiation (TBI). Animal studies demonstrated that Bu had a high level of myeloablative and antileukemic activity, but little immunosuppressive effect. In contrast, Cy was sufficiently immunosuppressive to allow allogeneic marrow engraftment in animal systems. In both animals and humans, the combination of Bu and Cy results in effective eradication of the host's bone marrow and suppression of the immune response, thus allowing engraftment by either allogeneic or autologous marrow. A number of BMT preparative regimens have been developed for use in leukemic patients. These regimens differ primarily in the myeloablative agent used (Bu or TBI) and the dose of Cy. Although few comparative studies have been conducted, BuCy regimens appear to be similar in efficacy to those containing Cy and TBI. In addition to its use in leukemic patients, BuCy BMT preparative regimens also have been used in patients with nonmalignant lymphohematopoietic disorders or other malignant diseases.