Two distinct histologic variants of primary breast osteosarcoma in 2 elderly women are described. The first patient was an 88-year-old woman with a long-standing, slow-growing, 18-cm mass in her right breast. The second patient was a 96-year-old woman with a recently self-detected, painless, 7.5-cm lump in her left breast. Clinically, there was no evidence of metastasis, and both women underwent simple mastectomy. Histologic features of both specimens were those of high-grade primary breast osteosarcoma. The first patient's tumor was classified as a chondroblastic variant, and the second as an osteoblastic variant of osteosarcoma. The patients were alive without evidence of local recurrence or hematogenous spread at a 16- and 4-month follow-up, respectively. Primary mammary osteosarcoma should be distinguished from metaplastic/sarcomatoid carcinoma with heterologous osseous/cartilaginous differentiation or malignant phyllodes tumor because it has a different biological behavior and requires a different treatment approach.