Extraskeletal osteosarcomas are rare. Few primary mammary osteosarcomas have been reported; many of these have been described in association with a biphasic tumor. Fifty pure osteosarcomas of the breast, diagnosed between 1957 and 1995, were reviewed after excluding those of biphasic origin. The absence of epithelial differentiation was confirmed using a panel of immunohistochemical markers in 32 cases and using ultrastructural evaluation in an additional four cases. Tumors occurred in 49 women and one man; age ranged from 27 to 89 years (median, 64.5 years). One patient received radiotherapy for ipsilateral breast carcinoma 9 years before presentation. Patients were treated by excisional biopsy (n = 13), tylectomy (n = 5) or mastectomy (n = 32). All axillary nodes, dissected in 20 patients, were free of tumor. One patient had extramammary spread at diagnosis. The neoplasms were 1.4 cm to 13.0 cm (mean, 4.6 cm), and 60% were grossly circumscribed. Tumors were classified as fibroblastic (n = 28), osteoclastic (n = 14), or osteoblastic (n = 8). Of 39 patients with available follow-up information, locally recurrent (n = 11) and metastatic (n = 15) disease developed in 23 (59%) at a mean of 10.5 and 14.5 months from diagnosis. Eight (73%) patients in whom local recurrence developed were treated with excisional biopsy or tylectomy; of these, seven had a positive margin. Sixteen (41%) patients died of disease at a mean of 17.1 months, within 20 months of onset of metastases, most commonly to the lung. Mammary osteosarcomas are aggressive tumors with a propensity for blood-borne rather than lymphatic spread. Total excision without axillary dissection is advised.