Among the 665 patients who registered at our hospital, we reviewed 39 cases of high grade primary osteosarcoma in patients who were older than 40 yr of age. The aim of this study was to determine if a primary osteosarcoma in older patients has different clinical features, and a poorer prognosis than in younger patients. Two evaluations were performed. In the first, an attempt was made to determine the possible prognostic factors such as gender, location, size, alkaline phosphatase, radiological findings, chemotherapy intensity, chemotherapy-induced tumor necrosis, and surgical margin. The second evaluation involved assessment of whether there were any significant clinical differences between older patients and adolescents. According to the results, a primary osteosarcoma in older patients did not reveal any significant prognostic variables. A primary osteosarcoma in older patients showed a poorer prognosis due to relatively unusual locations, common abnormal radiological findings, and a poor response to chemotherapy. Therefore, careful attention should be paid to making an accurate diagnosis and new strategies for more effective treatment, including chemotherapy, must to be developed in order to achieve long term survival in older patients with osteosarcoma.