Allogeneic bone marrow transplantation cures some persons with acute lymphoblastic leukemia, acute myeloblastic leukemia, and chronic myelogenous leukemia. Considerable data suggest that most cures result from immune-mediated antileukemia effects of the transplant rather than intensive pretransplantation chemotherapy and radiation. The mechanism of these immune-mediated effects, termed graft-versus-leukemia, is unknown. In the past 25 years more than 20,000 allogeneic transplantations were performed worldwide in persons with leukemia. Here we review single- and multicenter studies and analyses of the International Bone Marrow Transplant Registry on results of allografts for leukemia.