A case of extramedullary plasmacytoma (EMP) of the breast in a 49-year-old woman is reported. Six cases of EMP of the breast have been previously reported inthe world literature, but this case is unusual in that it was associated with serum IgD lamda monoclonal protein detected by serum immuno-electrophoresis. The protein was identical to the immunoprotein found in proliferating cells of the breast. Bone marrow aspiration revealed normal plasma cells indicating that the breast tumor did not represent extramedullary involvement of multiple myeloma or myelomatosis. An associated tumor was found in the nasal cavity after local treatment (mastectomy) of the breast tumor, but the patient remained disease-free 12months after adjuvant chemotherapy using nidlan, natulan, alkelan, prednisone, cyclophosphamide and vincristine.