Conventional myeloablative hematopoietic stem cell transplantation carries risks of morbidity and mortality from regimen-related toxicities that have restricted its use to relatively young patients in good medical condition. In nonmyeloablative allogeneic hematopoietic stem cell transplantation, enhanced immunosuppression rather than myeloablative cytotoxic conditioning has allowed the engraftment of allogeneic stem cells, with lower early transplant related mortality and morbidity. This approach shifts tumor eradication to the graft-versus-leukemia effect mediated by donor T lymphocytes. The development of nonmyeloablative transplantation has allowed the application of a potentially curative procedure to elderly or medically infirm patients who would not be able to tolerate high-dose conditioning regimens.