The authors report the case of a 66 year old woman, who underwent surgery for carcinosarcoma of the breast. This tumour consisted of an infiltrating ductal carcinoma intermingled with sarcomatous proliferation, resembling malignant fibrous histiocytoma, without evidence of any transition between the two. Carcinosarcomas of the breast are rare tumors. Their definition and their histogenesis are discussed in the literature. Their prognosis is worse than that of metaplastic carcinomas of the breast, particularly spindle cell carcinomas, justifying precise histological classification of these biphasic tumors.