Postradiation sarcoma of bone is an uncommon but serious sequela of radiation therapy. Seventy-eight Mayo Clinic patients have been treated for sarcomas arising in irradiated bones. They received their initial radiotherapy for a wide variety of nonneoplastic and neoplastic conditions, both benign and malignant. Thirty-five sarcomas arose in bone that was normal at the time of radiotherapy, and 43 arose in irradiated preexisting osseous lesions. The latent period between radiotherapy and diagnosis of sarcoma averaged 14.3 years. Ninety percent of the postradiation sarcomas were either osteosarcomas or fibrosarcomas; chondrosarcoma, malignant (fibrous) histiocytoma, malignant lymphoma, Ewing's tumor, and metastasizing chondroblastoma also occurred. Prompt radical surgery, when feasible, is usually the treatment of choice for the sarcoma. About 30% of patients with sarcomas of the extremities or craniofacial bones survived 5 years without recurrence; there were no disease-free survivors among patients with tumors of the vertebral column, pelvis, or shoulder girdle. The low risk of sarcoma following radiotherapy for the treatment of cancer should not be a contraindication to its use in these patients; however, radiation therapy for benign bone tumors should be reserved for lesions that are not amenable to surgical treatment. An unusual case is also reported herein in which a fibrosarcoma was discovered in the humerus of a patient who had received radiotherapy 55 years previously for a verified osteosarcoma in the same site.