Multiple myeloma of IgG kappa type was diagnosed in a 42-year-old man with bone pains, dyspnoea on exertion and increasing drowsiness. Six chemotherapy cycles extending over 14 weeks and consisting of 15 mg/m2 melphalan intravenously on day 1 and 60 mg/m2 prednisolone orally on days 1-4 produced a partial remission. As the HLA-identical sister of the patient was willing to donate bone marrow, an allogeneic marrow transplantation was planned. After 7 days' conditioning treatment (hyperfractionated whole-body irradiation with 12 Gy, chemotherapy with 70 mg/m2 melphalan and 60 mg/kg cyclophosphamide), 4.2 x 10(8) nucleated cells of donor marrow were infused per kg recipient body-weight through a central venous catheter. Despite prophylaxis with short-course methotrexate and cyclosporin, an acute graft-versus-host reaction of grade II-III occurred on day 26, though it settled almost completely after treatment with daily 2 mg/kg prednisone and monoclonal interleukin-2 receptor antibodies (B-B10, daily 10 mg). On day 100 after the marrow transplantation, marrow puncture showed the picture of complete remission with normal regeneration of haemopoietic cells. Allogeneic marrow transplantation may therefore be considered as a new and promising mode of treatment for younger patients with multiple myelomas.