We describe the clinical, radiological and pathological features of a series of six cases of primary fibromatosis of the breast. Most patients (5 of 6) were women of 22-58 years of age; one case occurred in a 47-year-old man. The fibromatosis of the breast in all cases presented as a monolateral solid nodule, clinically suspicious for malignancy and in three cases was also associated with cutaneous and/or nipple retraction. None of the patients was affected by any genetic disorder characterized by fibromatoses involving multiple sites, including breast. Both echographic and mammographic examinations revealed solid masses with irregular margins but without calcifications, mimicking scirrhous carcinoma. All cases were surgically treated by lumpectomy (4 of 6) or quadrantectomy (2 of 6), and after a follow-up period ranging from 5 months to 13 years all patients are well and disease-free. Histological examination revealed finger-like infiltrating margins entrapping adjacent breast parenchyma and adipose tissue in all cases. The hallmark of the lesion was the presence of bland-looking spindle cells, with a low mitotic index, organized in long sweeping and intersecting fascicles. Three different morphological phases (i.e. proliferative, involutional and residual), identical to those observed in Dupuytren's superficial fibromatosis, variably coexisted in each case. At immunohistochemistry the cells which comprised the lesion exhibited a diffuse expression of vimentin and a heterogeneous immunoreactivity to a-smooth muscle actin, thus confirming their fibro-myofibroblastic nature. No expression of estrogen, progesterone or androgen receptors or of pS2 correlated-estrogen protein was observed. We discuss both the pathogenesis of the mammary fibromatosis, pointing to the fibroblastic cells of the mammary stroma as the putative precursors, and the differential diagnosis versus the bland-looking, monomorphic spindle cell tumor-like or tumorous lesions of the breast.