Three cases of angiosarcoma of the breast are described. One case in a 66 year old woman was a solid mass and histologically showed poor differentiation leading to a death 14 months after the diagnosis. Another two in 46 and 25 year old females were large vascular masses showing moderate differentiation with endothelial tufting and papillary projections, and abdominal and contralateral mammary, and liver metastasis occurred within postoperative 10 months and 6 months in each case. A radical mastectomy and adjuvant chemotherapy in case 2 were not lifesaving. Although histologic degree of differentiation seems prognosis-related, angiosarcoma is the most malignant tumor of the breast.