Following the reactor accident at Chernobyl, this paper describes the biokinetics of radioiodine in man and discusses the radiation exposure resulting from intake of 131I. The risk of radiation-induced thyroid carcinomas and of congenital abnormalities is evaluated. Assuming a linear dose/risk relationship, one can calculate an increase in mortality from thyroid carcinomas amongst children in southern Germany of 100 to 101 per million children. For adults in southern Germany, and for the rest of the population in Germany, the figure is considerably lower. Gonadal dose from the 131I released is so small, compared with the annual natural radiation exposure, that it is not appropriate to discuss genetic effects.