Alarge irregular lobulated mass with heterogeneous density in the right breast and a smooth dense mass in the left breast were detected on computerized tomography of the chest of a 44-year-old Caucasian woman with history of palpable masses in bilateral breast (Fig. 1). A tru-cut biopsy revealed undifferentiated malignant tumor. Staging studies failed to show any distant metastases. With a presumed diagnosis of breast cancer, she underwent lumpectomy of the left breast and MRM of the right one. Right-sided mastectomy material contained a 15 · 10 · 8 cm tumor. Hypercellular tumor with osteoid deposits is seen in the lower half of the field. Normal breast ductal elements compressed by the tumor bulk are noted above (H&E, 100·) (Fig. 2a). Osteosarcomatous component with atypical osteoblastic proliferation, nuclear pleomorphism, and osteoid deposits (arrows) separated from compressed mammary parenchyma and ductal structures (H&E, 400·) (Fig. 2b). Epithelial ductal cells or remnants of epithelial elements were not identified on multiple sectioning. Malignant glandular and solid areas were not determined. Histopathologic examination revealed morphologic features of typical osteosarcoma. Left-sided lumpectomy material showed a 45 · 35 · 28 mm yellowish white solid lesion. Ductal elements with narrow lumens compressed in hypocellular uniform stroma are seen (H&E, 100·) (Fig. 3a). Breast ductules are seen in fibroblastic stroma consisting of fusiform cells with no cellular atypia and no mitoses (H&E, 200·) (Fig. 3b). Histologic diagnosis of benign cystosarcoma phylloides was made. After the diagnosis, she received adjuvant chemotherapy and radiation therapy. She is alive and being followed up without any evidence of recurrence for 28 months. The prevalence of primary breast sarcoma is less than 1% of women with breast malignancies. Breast osteosarcomas can arise from normal breast tissue or occur as a metaplastic differentiation of primary benign or malignant breast lesion. Also benign and malignant lesions like fibroadenomas, carcinomas may contain bone and osteoid cells. Metastatic bone formation may occur in fibroadenomas and phylloid tumors. After extensive sampling, neoplastic epithelial component can be found in these lesions. We presented our case as a primary breast osteosarcoma Address correspondence and reprint requests to: Ugur Coskun, MD, 40. Cadde. 444. Sokak, 5 ⁄ 5 Ebru Apt., Cukurambar Ankara, Turkey, or e-mail: ugurcos@hotmail.com.