Thyroid cancer is a well documented late effect of exposure to ionizing radiation. The excess risk begins 5-10 years after exposure and continues until at least 40 years after exposure. Females are roughly three times more susceptible to both radiogenic thyroid cancer and to thyroid cancer of other origins than are males. Therefore, relative risk estimates for radiogenic thyroid cancer do not necessarily differ by sex. The excess risk is higher among children exposed prior to five years of age than in those exposed later. The risk for radiogenic cancer following exposure to 131I appears to be lower than that following exposure to high dose-rate external irradiation, and in the Swedish diagnostic study 131I was nearly one fourth as efficient as external X-rays in inducing thyroid cancer. The Swedish data suggest that 131I is substantially less efficient in inducing thyroid cancer than high dose-rate exposures. In that study, however, 95% of the exposed individuals were 20 years or older (mean age 45 years).