Donor lymphocyte infusion (DLI), pioneered in Jerusalem in January 1987, represents the first proof of principle of the absolute efficacy of immunotherapy as a means of curing cancer. Immunotherapy with alloreactive donor lymphocytes can eliminate "the last tumor cell" even in patients with hematological malignancies resistant to maximally tolerated doses of chemoradiotherapy. Alloreactive lymphocytes that can mediate anti-tumor effects following induction of host-versus-graft tolerance induced by transplantation of donor stem cells, can induce graft-versus-malignancy (GVM) effects which are usually accompanied by graft-versus-host disease (GVHD). However, occasionally GVM effects may also be accomplished independently of clinically overt GVHD. Interestingly, allogeneic donor lymphocytes may also eliminate undesirable host-derived hematopoietic cells in a large number of nonmalignant indications including genetic diseases, diseases caused by deficiency of stem cell products, and autoimmune disorders mediated by self-reactive lymphocytes. The cumulative clinical experience suggests feasibility of effective induction of graft-versus-leukemia (GVL); graft-versus-lymphoma (GVLy); graft-versus-multiple myeloma, as well as graft-versus-solid tumors (GVT), well-documented in patients with renal and breast cancer, even in patients with resistant disease that have failed myeloablative chemoradiotherapy. These observations that suggested that cell therapy by donor lymphocytes is the main therapeutic benefit of bone marrow transplantation (BMT) led to development of the nonmyeloablative approach for safer allogeneic stem cell transplantation. Nonmyeloablative stem cell transplantation (NST) makes it possible to offer an option for cure to elderly patients with no upper age limit, as well as to patients with poor performance status not considered eligible for conventional BMT. Using well-tolerated NST regimen, allogeneic stem cell transplantation can be accomplished with minimal procedure-related toxicity and mortality, possibly even on an outpatient basis. Immunotherapy mediated by adoptive allogeneic cell-mediated immunotherapy can be further improved by utilizing specifically immune donor lymphocytes, thus maximizing their efficacy against undesirable target cells of host origin on the one hand, while minimizing their ontoward efficacy against normal cells of host origin that could result in GVHD on the other. Taken together, DLI and subsequently NST, may have opened new horizons for treatment of life-threatening malignant and nonmalignant disorders correctable by allogeneic stem cell transplantation. It is anticipated that further improvement of reactivity and specificity of donor lymphocytes will lead to safer clinical application of cell therapy for a larger number of indications toward improving disease-free survival in a large number of indications while minimizing immediate and late procedure-related complications.