Thirty-four additional cases of sarcoma developing in previously irradiated bone are documented, bringing the total Mayo Clinic experience in sarcoma appearing after radiation therapy to fifty-five instances. The sarcomas arose in normal bone within the radiation treatment field in eleven patients, while they arose in pre-existing bone lesions in sixteen patients. Osteogenic sarcoma and fibrosarcoma were most commonly diagnosed. The outlook in tumors of this nature is poor, the average survival being 1.14 years, with only two long-term survivors. Although a relationship has been established in experimental animals, a direct relationship between exposure to external radiation and the development of sarcoma is not established in man. There is no absolute proof that ionizing radiation caused any of these sarcomas. However, this and other significantly large series add to the mounting circumstantial evidence that the association between ionizing radiation and sarcoma is a real one. It must also be emphasized that this is a rare complication clinically.