Of 1000 osteosarcoma cases recorded in the Mayo Clinic records, approximately 7% of the lesions occurred in the diaphysis of long bones, mostly in the femur, tibia, and humerus. The average age of the patients with these lesions was 28 years (range: 8 to 67). The duration of their symptoms before diagnosis was greater than that for patients with conventional osteosarcoma in the metaphysis. In approximately 20% of the cases, the radiographic appearance mimicked that of Ewing's sarcoma. Amputation surgery was the major form of treatment. The 2-, 5- and 15-year survival was 42%, 29%, and 20%, respectively.