Fibromatosis of the breast is an uncommon benign disease that mimics carcinoma clinically as well as radiographically. Its microscopic diagnosis among spindle cell tumors is fundamental, especially on frozen sections. We report two cases of mammary fibromatosis observed in young woman (26 and 31 years old). The two were clinically and mammographically suspect. Diagnosis was made on frozen sections and confirmed on paraffin embedded specimen. The lesions are characterized by spindle cell proliferation of variable cellularity and collagenization. Few mitotic figures were present in one case. Several lesions must be considered in the differential diagnosis of mammary fibromatosis. Wide local excision is recommended for initial therapy, but local recurrence is frequent. In our cases, In spite of apparently complete excision, one patient relapsed.