A case of primary malignant fibrous histiocytoma of the breast is reported. The patient was a 48-yr-old woman with a huge tumor involving almost the entire left breast. The central portion of her left breast was already rotted by extensive necrosis and inflammation. She was treated by radical mastectomy and axillary lymphadenectomy to level I. Pathologic examination supported by an immunohistochemical staining confirmed the tumor as malignant fibrous histiocytoma of giant cell type. Axillary lymph nodes were free from tumor metastasis. She had not taken any postoperative adjuvant therapy. The metastasis to lungs was found 2 months after the operation, and she died within 6 months.