Multiple myeloma (MM) is refractory to conventional chemotherapy. To achieve a sustained complete remission, we performed planned non-myeloablative allogeneic stem cell transplantation (NST) after autologous hematopoietic stem cell transplantation (HSCT) in a patient with stage III MM. Autologous HSCT was performed using high-dose melphalan after conventional chemotherapy, followed by NST from an HLA-identical sibling using low-dose total body irradiation (200 cGy) for conditioning. Cyclosporine and mycophenolate mofetil were used for graft-vs-host disease (GVHD) prophylaxis. Acute GVHD was transiently seen in the skin and intestine, while, in addition, mild chronic GVHD was seen in the oral mucosa and skin. Complete donor chimerism was achieved and the disappearance of tumor-derived monoclonal B cells was confirmed based on an analysis of immunoglobulin light chain messenger signals on day 156 when chronic GVHD occurred. The clinical course in this case strongly suggested the existence of a graft-vs-myeloma effect.