Osteosarcoma of the breast is an unusual neoplasm. The authors describe such a tumour found in a 38 year old woman. The neoplasm developed rapidly within 5 months of the patient's first observation. Histological study revealed extensive fibroadenomatous tissue with osteosarcomatous metaplastic foci. The tumor rapidly recurred and was then composed exclusively of sarcomatous tissue. The patient died 15 months after first recognition of the tumor probably from generalized metastases. The epithelial and mesenchymatous origin of osseous metaplasia within (phylloid) fibroadenoma is discussed.