Donor lymphocyte infusions for treatment of relapse after allogeneic bone marrow or stem cell transplantation is being used with increasing frequency. Study of this form of adoptive immunotherapy will shed light on different aspects of cell-mediated cytotoxicity such as antigen presentation, processing, immune recognition, lymphocyte subsets involved, and mechanism of cell death. Donor lymphocyte infusions are extremely effective for cytogenetic relapses or chronic-phase relapses of chronic myelogenous leukemia, but are less effective in acute leukemias or other disorders. Donor lymphocyte infusions are associated with a significant risk of morbidity and mortality due to graft-versus-host disease and pancytopenia. Lower cell doses, earlier infusions, and selective depletion of CD8+ lymphocytes have been proposed as methods of diminishing these toxicities. Current research is focusing on methods of making donor lymphocyte infusions more effective in the nonchronic myelogenous leukemia setting, and decreasing their toxicity without losing their clinical efficacy in the treatment of relapsed chronic myelogenous leukemia.