Patients with malignant and non-malignant hematologic diseases including severe aplastic anemia (SAA), paroxysmal nocturnal hemoglobinuria (PNH), myelodysplastic syndrome (MDS), acute and chronic leukemias, Hodgkin's and non-Hodgkin's lymphoma and multiple myeloma (MM) can now be cured by allogeneic bone marrow transplantation (BMT). This curative effect has been ascribed to the use of high dose chemo-radiotherapy and the anti-tumor or anti-bone marrow effect of the allograft. Dose intensification of conditioning regimens in attempts to reduce disease recurrence has been largely unsuccessful because of increased toxicity and mortality. Indeed, most evidence now points to donor-derived T-cells as being the principal modality leading to the complete eradication of both malignant and non-malignant host hematopoietic cells.