Allogeneic hematopoietic stem cell transplantation (HSCT) is the most effective treatment for selected hematological malignancies. Its curative potential is largely mediated by an immune-mediated destruction of malignant cells by donor lymphocytes termed graft-versus-leukemia (GVL) effect. However, because of its toxicity, conventional allogeneic HSCT is restricted to younger and fitter patients. These observations led several groups to set up new (less toxic) transplant protocols (nonmyeloablative stem cell transplantation or NMSCT) based on a two-step approach: first, the use of immunosuppressive (but nonmyeloablative) preparative regimens providing sufficient immunosuppression to achieve engraftment of allogeneic hematopoietic stem cells and, in a second step, destruction of malignant cells by the GVL effect. Preliminary results showed that NMSCT were feasible with a relatively low transplant-related mortality (TRM), even in patients older than 65 years. In addition, strong antitumor responses were observed in several hematological malignancies as well as in some patients with renal cell carcinoma. After discussing the mechanisms and efficacy of the GVL effect as well as the rationale for NMSCT strategies, this article reviews the first results of ongoing clinical trials. Innovative modalities that may permit amplification of the GVL effect while minimizing the risk of GVHD are discussed. Because the benefits of NMSCT over alternative forms of treatment remain to be demonstrated, this strategy should be restricted to patients included in clinical trials.