Adoptive immunotherapy with donor-derived buffy coat cells for relapsed hematological malignancies after allogeneic BMT is an established and highly effective treatment. We report a patient who relapsed on day +330 after allogeneic sibling BMT for multiple myeloma with multiple solid subcutaneous tumors consisting of plasma cells. Histology and immunocytology of the bone marrow did not show plasma cell infiltration. After cessation of the immunosuppression consisting of cyclosporine and methylprednisolone, a total of 6.2 x 10(7)/kg recipient body weight CD3+ T cells derived from the donor by leukapheresis were transfused on 4 consecutive days. To enhance the T cell effect six doses of 5 million units alpha interferon were given subcutaneously. Five days later the tumors started to shrink and have completely vanished since day x400 after BMT. The patient developed acute GVHD grade III of the liver and gut which was treated by reinduction of various immunosuppressive drugs. Up to now there is no evidence for relapse of the multiple myeloma, but the patient suffers from extensive chronic GVHD (gut and liver). This is the first report to demonstrate a graft-versus-myeloma effect for relapse with solid tumor manifestation after sibling BMT with donor-derived buffy coat cells as adoptive immunotherapy.