This article discusses changes in the way hematopoietic stem cell allotransplants may be carried out in the future to treat patients with malignant hematological diseases. Specifically, the focus has shifted away from attempts at eradicating underlying diseases through toxic high-dose chemoradiation therapy towards using the stem cell donor's immune cells for that purpose (allogeneic graft-versus-tumor effect). The non-myeloablative transplant approaches hold promise in reducing the morbidity and mortality associated with conventional high-dose chemoradiation therapy, and they allow allogeneic transplants in elderly or medically infirm patients who are at present not candidates for transplantation. In the future, specific graft-versus-tumor responses may become possible by eliciting donor T cell responses to tumor-associated minor histocompatibility antigens. In Section I, Dr. Rainer Storb describes experimental studies in random-bred dogs that rely on non-cytotoxic immunosuppressive agents to establish stable allografts. Powerful postgrafting immunosuppression, traditionally directed at preventing graft-versus-host disease (GVHD), is also used to overcome host-versus-graft (HVG) reactions, thereby dramatically reducing the need for intensive immunosuppressive conditioning programs. Preclinical canine studies have been translated into the clinical setting for treatment of elderly or medically infirm patients with malignant hematological diseases. The pretransplant conditioning has been reduced to a single dose of 2 Gy total body irradiation (TBI) with or without fludarabine. The lack of toxicity makes it possible for transplants to be conducted in the outpatient setting. Multicenter trials have been initiated, and more than 300 patients have been successfully treated with hematopoietic stem cell grafts both from related and unrelated HLA-matched donors. In Section II, Dr. Richard Champlin describes clinical studies with therapeutic strategies that utilize relatively non-toxic, nonmyeloablative disease-specific preparative regimens incorporating fludarabine, together with other chemotherapeutic agents, to achieve disease suppression and engraftment of allogeneic hematopoietic cells and to allow subsequent infusions of donor lymphocytes. Remissions have been seen in patients with acute myelocytic, chronic myelocytic, chronic lymphocytic, leukemias, lymphomas, and myelomas. In Section III, Dr. Stanley Riddell and colleagues describe studies on isolation of T cells reactive with minor histocompatibility (H) antigens and involved both in GVHD and graft-versus-leukemia (GVL) responses. For example, the gene encoding a novel H-Y antigen in humans has been identified and shown to exhibit restricted tissue expression. Acute myelocytic leukemia stem cells were demonstrated to express the H-Y antigen and additional minor H antigens, and engraftment of such cells in NOD/SCID mice could be selectively prevented by minor antigen-specific T-cell clones. An autosomal encoded human minor H antigen associated with chronic GVHD has been demonstrated. A trial evaluating therapy of relapsed acute myelocytic leukemia or acute lymphoblastic leukemia after allogeneic stem cell transplantation with T-cell clones specific for recipient minor H antigens has been initiated.