The results of donor lymphocyte infusion (DLI) for treatment of relapse after bone marrow transplantation (BMT) are reviewed. Durable complete remission can be achieved at the molecular level for a majority (more than 70%) of patients with CML, when treated at early relapse. Results are less favourable for acute leukemias, although useful responses have been reported. Data are scarce though promising for myelodysplastic syndromes and multiple myeloma. Major treatment-associated toxicities are GVHD and bone marrow aplasia. The latter complication can be predicted by evaluating the level of residual donor-derived hematopoiesis. Modification of infused cells (CD8 negative selection or transduction with a suicide gene), addition of peripheral blood stem cells, and early implementation of escalating doses may counteract the complications and increase the response rate. Response rate is variably influenced by the presence of chronic GVHD after initial BMT, T-cell depleted BMT, underlying disease and stage at relapse, and the level of mixed chimerism. DLI is a direct demonstration of the graft-versus-leukemia effect (GVL). Because GVL after BMT is sometimes the predominant cause of cure, it may be advisable in such situations to redirect the conditioning regimens for BMT towards engraftment and less immediate cytotoxicity.