The use of supralethal chemoradiotherapy followed by marrow transplantation has progressed from being an experimental approach applied only to a limited number of end-stage patients to an important therapeutic option appropriate for many adults with a variety of hematologic malignancies. With the use of transplantation, 10% to 30% of patients with relapsed leukemia and approximately 50% of patients with acute nonlymphoblastic leukemia in first remission can be cured. Cures have also been seen in a variety of other hematologic malignancies, including chronic granulocytic leukemia, preleukemia, hairy cell leukemia, and malignant lymphoma. Transplantation is currently limited by the need for a suitable marrow donor; by the complications of the transplant procedure, including infection, graft-versus-host disease, and the toxicities of intensive chemoradiotherapy; and by the risk of recurrent disease. Some of these limitations will likely be overcome as a result of current research. The use of partially matched family members and matched unrelated donors will make transplantation available to more patients. Some forms of posttransplant infection, including those associated with herpes simplex and cytomegalovirus, can now be prevented or treated. Improved methods of controlling graft-versus-host disease including T-cell depletion of marrow and the use of more effective immunosuppressive agents, as well as a better understanding of the toxicities of the preparative regimens, are making the transplant procedure safer and more tolerable. Finally, the development of better preparative regimens and transplantation earlier in the patient's disease course will likely allow for a larger percentage of patients to be cured.