A 69-year-old woman was diagnosed with a malignant tumor of the right proximal femur. She had primary hyperparathyroidism and chronic elevation of parathyroid hormone levels (PTH > 1,000 pg/ml). She underwent resection of the bone lesion; histological analysis showed a high-grade fibroblastic osteosarcoma. In addition, she underwent curative resection of a large left superior parathyroid adenoma. To our knowledge, this is the third reported clinical case of osteosarcoma arising in association with hyperparathyroidism.