Metaplastic breast carcinoma, a rare entity, accounts for only about 0.02% of all breast carcinomas. It involves the transformation of mammary neoplasms into osteoid and chondroid substances. Because of the relatively small patient population and the limited number of controlled studies, there is confusion regarding its classification and staging. Its histogenesis is unknown. The authors describe a 65-year-old woman with findings consistent with metaplastic breast carcinoma. Theories as to etiology and prognosis as well as treatment are discussed.