A 50-year-old woman had a malignant tumor of the left breast, which recurred twice, metastasized, and caused death after 39 months. Histologically, the original neoplasm and the first recurrence comprised an adenomyoepithelioma, in addition to a sarcoma composed of trabeculae of mature and immature bone, osteoid, and partly calcified, dense collagenous tissue. The trabeculae were lined by alpha-smooth muscle actin-positive mononuclear tumor cells, which also extended into the stroma. Similarly, scattered osteoclastlike, multinucleate giant cells were present in the stroma and in the region of the trabeculae. This same pattern of adenomyoepithelioma and osteosarcoma also was seen in the last recurrence, together with a proliferation of undifferentiated malignant spindle-shaped cells. The last biopsy also contained a separate small focus of invasive ductal carcinoma of usual type. It was concluded that this, apparently unique, tumor probably represented an adenomyoepithelioma in which a metaplastic sarcoma of osteogenic and undifferentiated types developed from the myoepithelial element, and in which a carcinoma developed from the epithelial component.