Some of the recent advances in our knowledge of immune recognition have provided new tools to circumvent or reverse some of the major disadvantages of allogeneic bone marrow transplantation (BMT). The pretransplant conditioning regimen produces a major defect in the immune system that greatly favors the occurrence of life-threatening infections, caused particularly by Epstein-Barr virus and cytomegalovirus. However, adoptive transfer of virus-specific cytotoxic T lymphocytes can reconstitute specific immunity and/or cure viral disease in immunocompromised post-BMT patients. The other major drawback of allogeneic BMT is graft-versus-host disease (GVHD). Although potentially detrimental, it is closely associated with an antileukemia reaction (graft-versus-leukemia, GVL). The most direct evidence of the GVL effect has been provided by the efficacy of donor leukocyte infusions (DLI). DLI can induce long-lasting remissions, especially in patients with chronic myeloid leukemia who relapse post-BMT. Although allogeneic cell therapy should still be considered a "naive" form of immunotherapy, work in progress on the identification of leukemia-specific antigens will improve the outcome and enlarge its applications.