Four cases of so-called carcinosarcoma of the breast were studied histopathologically. The patients were females aged 44, 37, 44 and 77 who were admitted to the Cancer Institute Hospital for breast tumors. Histologically, the tumors showed proliferation of sarcomatous cells with relatively small amounts of papillotubular carcinoma or scirrhous carcinoma. There were transitions between the sarcomatous portion and carcinoma. Two cases showed squamous metaplasia merging into the sarcomatous component, whereas the other 2 showed transition from adenocarcinoma to sarcomatous element. Lymph nodal metastases in 2 cases were papillotubular carcinoma. One patient died of lung and brain metastases, and autopsy revealed only sarcomatous element.