A 19-year-old man with a high-grade osteosarcoma of the femur, treated with neoadjuvant chemotherapy, surgery, and adjuvant chemotherapy suffered from lung metastases 15 months after diagnosis. They were resected. Thirteen months later, he had vague abdominal complaints which, after analysis, proved to be caused by peritoneal metastasis. A review of the literature, possible physiopathological mechanisms of increased occurrence of unusual metastases and the role of bone scintigraphy in the follow-up of patients with osteosarcoma are discussed.