Allogeneic stem cell transplantation is a curative treatment modality successfully employed in a number of hematologic malignancies. The curative effect of this approach is in part mediated by donor-derived T lymphocytes with reactivity for patient leukemic cells. The power of this graft versus leukemia effect (GVL) is best illustrated in patients with relapsed Chronic Myeloid Luekema (CML) after an allogeneic bone marrow transplant, in whom a single donor lymphocyte infusion (DLI) can induce remission. We hypothesize that neoplastic T cells in MF/SS may similarly be susceptible to a graft vs. tumor (GVT) effect. Unfortunately, advanced patient age and a 25% to 35% risk of transplant related mortality (TRM) preclude the use of conventional 'dose- intensive' allogeneic peripheral blood stem cell transplantation (PBSCT) in patients with advanced CTCL who might otherwise benefit from this approach. The risk of TRM related to conditioning can be circumvented at least partially by using a reduced-intensity conditioning regimen to prepare the patient for transplantation.