A 65-year-old woman noticed a left breast mass and underwent excisional biopsy. The excised mass measured 1.7 cm in diameter. Histologically, the tumor-like mass revealed irregular vascular channels with occasional thrombi and many papillary projections of fibrous stalks lined with a layer of flat endothelial cells. A small cavernous hemangioma was present adjacent to the lesion. Scanning electron microscopic observation revealed disorderly oriented thin and thick cords forming branches and meshworks in the lumina. The luminal wall and cords were covered with flat endothelial cells. Light and scanning electron microscopic differences between intravascular papillary endothelial hyperplasia and angiosarcoma are discussed.