A case of fibromatosis of the breast mimics a carcinoma by its clinical and mammographic features. The morphological image of this rare disease of the breast is described. The fibromatosis of the breast has to be distinguished histologically from a fibroma and from a low-grade fibrosarcoma. The dignity of the lesion is confirmed by histological and immunohistochemical examination. The possibilities of therapy are discussed and compared with data of the literature.