A review has been made of clinical aspects of sarcoma induced by external irradiation. The literature is surveyed and two new cases are reported. One arose in an area of chronic radiodermatitis over the sacrum 7 years after million-volt X-ray therapy for papilloma of the bladder. The other, an osteogenic sarcoma, occurred in the mandible 7 years after deep X-ray therapy for facio-cervical actinomycosis. Irradiation sarcoma is a clinical rarity which may supervene when connective tissue is subjected to high dosage. Bone is likely to be most often affected because of its high equivalent atomic number and because it is also the commonest site of spontaneous sarcoma. A latent period of years elapses before malignancy is seen (for soft-tissue sarcoma, 5 to 30 years; for bone sarcoma, 3 to 22 years: mean 8·6 years). For superficial sarcoma the dosage necessary is of the order which produces chronic radiodermatitis. For bone sarcoma it is considered to be above 3000 r. Supervoltage therapy enables deep tissues to be heavily irradiated leaving relatively little skin change, and this factor may require consideration in the diagnosis of cases in the future. The role of co-existing inflammation in pathogenesis is discussed. Treatment of irradiation sarcoma is essentially surgical, but useful growth-restraint may be obtained in inoperable cases by radiotherapy. The practical implications include the inadvisability of irradiating benign lesions in and around bone at high dosage.