Fewer than 1% of malignant breast tumors are sarcomas. Lack of correlation between the biologic behavior and histopathologic appearance of these tumors has been reported. A well-differentiated fibrosarcoma could be mistaken for breast fibromatosis, which may be a familial trait and may occur in Gardner syndrome. Forty-three cases of mammary fibromatosis have been reported. Most authors are concerned with misdiagnosis of breast fibromatosis as a malignant lesion leading to unnecessary mastectomy. However, the result can be disastrous when a well-differentiated fibrosarcoma is diagnosed as fibromatosis and the patient develops lung metastases.