Bone marrow transplantation can cure otherwise lethal diseases, improve quality of life, and prolong survival for many patients. More than one half of acute nonlymphocytic leukemia and chronic myelogenous leukemia patients and one third of acute lymphocytic leukemia patients in early remission who have matched sibling donors clearly benefit from such therapy. Advances in transplantation technology may extend benefits to patients lacking sibling donors. Continued investigation into fundamental immunology and tumor biology, as well as careful attention to the development of improved supportive-care techniques to reduce both the risks and the costs of marrow grafting, are still required. The complexities of such therapy are enormous, but equally impressive are the clinical successes already achieved. Continuing advances in this exciting field are sure to come.