The cardiac complications of radiotherapy have been reviewed in the light of 46 case histories collected from 16 treatment centres in France. They are usually found after radiotherapy for Hodgkin's disease and carcinoma of the breast. Although histologically there may be diffuse fibrosis involving the endocardium, myocardium and pericardium, it is pericarditis which is the usual clinical feature. It often occurs late, and takes many forms. The dry constrictive fibrous type of pericarditis is the most severe because of the amount of myocardial fibrosis usually associated with it. It is rare for there to be clinical features of myocardial fibrosis, and if present they are variable: isolated disorders of repolarisation or of conduction, or true cardiomyopathies. Lesions of the coronary arteries are exceptionally rare after radiotherapy, and involve fibrosis of the intima or atherosclerosis.