Fibromatosis is a relatively benign, though locally invasive neoplasm. About 100 cases of fibromatosis of the breast have been reported to date, none of them in a male. We present a 60-year-old man who was admitted to the University Hospital for Tumors in Zagreb, Croatia, on May 25, 1992, with a 3-month history of gradually enlarging, painless subcutaneous mass, 6 x 5 cm, that was located in the upper outer quadrant and partly beneath the areola of his left breast. The diagnosis was confirmed by immunoperoxidase studies, that showed positivity for vimentin, whereas the staining for cytokeratin AE1, glial fibrillar acid protein (GFAP) and collagen IV was negative. The tumor was removed by en bloc resection, and the greater pectoral muscle was spared. Today, 3 years after the operation, the patient is alive and in good health.