A 72-year-old black woman presented with huge enlargement of the right breast beginning approximately 30 years earlier and progressively enlarging over the years. The breast was stony hard, painless, and inconvenient only because of size and weight. Simple mastectomy was done and pathologic examination showed a benign mixed tumor with large areas of pseudocartilage, osteoid, and bone formation. Preoperatively the serum calcium was 11.0 mg/100 ml and alkaline phosphatase, 550 mU/ml (SMA-12). Postoperative course was uneventful and within several weeks the alkaline phosphatase fell to a normal range (35-115 mU/ml). The case presented requires little comment other than its unusual nature and duration. Extensive bone formation was undoubtedly responsible for the elevated alkaline phosphatase and, as expected, it returned to normal limits following excision of the tumor. Simple mastectomy is considered the treatment of choice.