A case of radiation-induced sarcoma of the chest wall is reported. Twenty-seven years 11 months after orthovoltage radiotherapy of the right breast a 69-year-old woman developed a radiation-induced osteosarcoma of the right thoracic wall. Initial diagnosis has been T-cell lymphoma of the skin. The right breast was irradiated with tangential fields and a total dose of 40 Gy, 2 Gy/day, 5 days a week. Orthovoltage treatment was performed in two courses of 20 Gy, 3 months apart. The clinical appearance of the secondary sarcoma was a diffuse infiltrated area in the irradiated breast which seemed to be fixed to the chest wall. Magnetic resonance imaging (MRI) demonstrated a mass in the right anterior thoracic wall which destroyed the fourth to the sixth rib. The tumor infiltrated the thoracic wall, including subcutaneous tissue and pericardium, as well as extending into the subphrenic space. Biopsy of the lesion revealed a poorly differentiated osteosarcoma. The patient's general condition precluded surgical or chemotherapeutic intervention; she died due to a cerebral stroke 6 months later. This case fulfilled all criteria for radiation-induced sarcoma, as there was a prior history of radiotherapy, latency period of several years, development of sarcoma within the irradiated field, and histologic confirmation of sarcoma.