The considerable increase in the incidence of thyroid cancer in children in areas exposed to the fallout from the Chernobyl nuclear reactor accident has drawn attention to the need for a better understanding of the relationship between radiation exposure (especially from the radionuclides of iodine) and the risk of thyroid cancer. An increase in thyroid cancer has been reported both in patients exposed to therapeutic and diagnostic external radiation, and in the population exposed to radiation from the Hiroshima and Nagasaki atomic bombs. While there is no evidence of a significant increase...