A case of myofibrosarcoma of the breast is described. A 55-year-old woman presented with a small mammary nodule. A large recurrent lesions appeared a month later, and the patient died 11 months following initial presentation from diffuse pleuropulmonary metastases. Histologically, the primary tumor consisted mainly of spindled cells, arranged in fascicles and surrounded by varying quantities of dense hyaline collagen. The recurrent lesion had a more pleomorphic organization. In both lesions, there was positive immunostaining for vimentin, smooth-muscle actin, and fibronectin, and negative results for desmin, laminin, and type IV collagen. Electron microscopy revealed abundant rough endoplasmic reticulum, myofilaments with focal densities, and the fibronexus junctions and fibronectin fibrils characteristic of myofibroblasts. Given these cellular features and behavior, the tumor was interpreted as a malignant neoplasm showing myofibroblastic differentiation, i.e., a myofibrosarcoma. This case enlarges the group of myofibrosarcoma of breast, also with the demonstration of fibronexus and fibronectin fibrils. The paper emphasizes the criteria required for myofibroblastic differentiation and reviews lesions of the breast reported in the literature as myofibroblastic.