A 72-yr-old acromegalic man, who presented with pain in the left femur, was found to have a metastatic osteosarcoma. Only three cases describing the coexistence of acromegaly and osteosarcoma have been reported by the literature. As the patient didn't have other risk factors for osteosarcoma, the hypothesis that accelerated rate of bone turnover caused by long-term exposure to high GH and IGF-I could act as a predisposing factor in the development of this malignant bone tumor is discussed.