Fibromatosis (desmoid tumors) is a rare entity expressing a mesenchymatous proliferation. The breast is an exceptional localisation. The origin is either the gland itself or the breast is an extension of a desmoid tumor originating from the chest wall or shoulder. Etiology is still unknown. Clinical and radiological features mimic the breast carcinoma. Diagnosis is made by histological studies which show a benign structure contrasting with its aggressive potential. Surgery is the treatment of choice and consists of wide excision which can lead to mutilation. Hormonotherapy is used if surgery is too invasive. We report a case of a female patient aged 21 years who presented for a mammary fibromatosis, treated by a large surgical excision. Evolution was marked by the appearance of an other localisation in the contralateral breast.