Several developments have improved disease-free survival after allogeneic bone marrow transplantation. They have mainly involved prophylactic and therapeutic interventions to reduce some of the transplant-related complications. It is now apparent that disease control is achieved by several mechanisms, including the preparative regimen as well as immunologic interactions between tumor cells and cells that are graft derived and belong to the immune surveillance system. Appropriate manipulations of the latter group of mechanisms may result in a better understanding of disease control and make the underlying therapeutic principle universally applicable. Chronic myeloid leukemia patients in the chronic phase appear to have the most optimal risk-to-benefit ratio of all patients transplanted for hemopoietic malignancies. More recent results in acute myeloid leukemia patients transplanted in first remission, however, suggested that allogeneic bone marrow transplantation might also be acknowledged as the treatment of choice in this disease. This conclusion cannot be drawn as yet for patients with acute lymphoblastic leukemia, chronic lymphocytic leukemia, and malignant lymphoma.