Allogeneic bone marrow or blood stem cell transplantation (BMT) represents an important therapeutic tool for the treatment of an otherwise incurable broad spectrum of malignant and non-malignant diseases. Until recently, BMT was used primarily to replace a malignant, genetically abnormal or deficient immunohematopoietic compartment and therefore, highly toxic myeloablative regimens were considered mandatory for more effective eradication of all undesirable host-derived hematopoietic cells, including stem cells and their progeny. Our preclinical and ongoing clinical studies indicated that much more effective eradication of host immunohematopoietic system cells can be mediated by donor lymphocytes in the process of adoptive allogeneic cell therapy following BMT. Thus, eradication of all malignant cells, especially in patients with CML and, to a lesser extent, in patients with other hematologic malignancies can be accomplished despite complete resistance of puch tumor cells to maximally tolerated doses of chemoradiotherapy. Our cumulative experience suggested that graft-versus-malignancy effects might be used as a tool for eradication of otherwise resistant tumor cells of host origin. We speculated that the therapeutic benefit of BMT may be improved by using safer conditioning for engraftment of donor stem cells induce host-versus-graft unresponsiveness to enable engraftment of donor lymphocytes for subsequent induction of graft-versus-malignancy effects, or even graft-versus-autoimmunity and graft-versus-genetically abnormal cells. In other words, focusing on more selective and smarter rather than stronger modalities. Effective BMT procedures may be accomplished without lethal conditioning of the host, using a new, well-tolerated and user-friendly non-myeloablative regimen, thus eliminating or minimizing immediate and late procedure-related toxicity and mortality. It appears that initial induction of graft tolerance, mediated by engraftment of donor stem cells, leads to durable engraftment of immunocompetent donor lymphocytes, which may be necessary for induction of effective biologic warfare against host-type immunohematopoietic cells. Consequently, stem-cell therapy following induction of transplantation tolerance by selective elimination of alloreactive donor lymphocytes may represent the treatment of choice for a wide range of otherwise incurable diseases, including cancer (hematologic malignancies and certain metastatic solid tumors), genetic disorders (hemoglobinopathies and enzyme deficiency disorders), diseases caused by self-reactive lymphocytes (autoimmune diseases such as multiple sclerosis, rheumatoid arthritis) to mention just a few. Using reduced intensity conditioning, non-myeloablative stem cell transplantation (NST) can be accomplished with no major procedure-related toxicity or mortality. Thus, NST offers the feasibility of safe stem cell transplantation and cell-mediated procedures for a large and constantly growing spectrum of clinical indications for all patients in need without lower or upper age limit. Future strategies currently under investigation include developing new approaches for control of alloreactivity of host-versus-graft and graft-versus host reactivity reactions and developing better approaches for maximizing the capacity of donor lymphocytes to eliminate cancer cells more selectively, while avoiding or minimizing GVHD for safer and more effective treatment of patients in need of BMT.