A case of secondary plasmacytoma (Ig. D. lambda) of the breast, associated with multiple myeloma, is reported, involving a 60-year-old woman in whom multiple tumors of the right breast appeared during the course of her treatment for multiple myeloma. These breast tumors were firm, non-tender, clear-margined, smooth-surfaced, and showed good mobility. An ultrasonogram revealed that the tumors had the shape of a clear-margined anechoic mass like a cyst. A histological diagnosis of a biopsied specimen determined plasmacytoma (Ig. D, lambda) and the patient was treated with chemotherapy which caused the tumors to greatly reduce.