Primary osteogenic sarcomas of the breast are exceptionally uncommon. We describe such a case occurring in a 66-year-old woman who presented with a hard mass in her left breast. Mammography and breast ultrasonography showed a calcified breast lump, but features were not diagnostic. Modified radical mastectomy of the left breast, including axillary lymph node dissection, was performed. Microscopical and immunohistochemical findings established the diagnosis of primary osteogenic sarcoma. Because there was no evidence of metastasis, no further treatment was considered necessary. She remained well 15 months later, without tumour recurrence. We discuss in detail the diagnostic implications of this rare entity.