Immunotherapy of cancer with alloreactive lymphocytes is the mainstay of treatment, especially in haematological malignant disease. With donor lymphocyte infusion for immunotherapy, it is essential to induce host-versus-graft tolerance to ensure that the donor lymphocytes are accepted. Engraftment of haemopoietic cells of donor origin can be accomplished with reduced-intensity conditioning. Reducing transplant-related mortality by simplifing the stem-cell transplant procedure with a reduced-intensity regimen, particularly non-myeloablative conditioning, may have great potential for the treatment of malignant and non-malignant disorders.