During the past few years, the United Nations Scientific Committee on the Effects of Atomic Radiation1 has undertaken broad reviews of the sources and effects of ionizing radiation. Estimates of the carcinogenic risk from exposures to ionizing radiation were presented in the UNSCEAR 2000 Report2 . In its 2001 Report, the Committee has completed a comprehensive review of the risks to offspring (hereditary risks) following parental exposure to radiation. For the first time, the review included an evaluation of those diseases which have both hereditary and environmental components, the so-called multifactorial diseases. The major finding is that the total hereditary risk is 0.3 0.5 per cent per gray to the first generation following radiation. This is less than one tenth of the risk of fatal carcinogenesis following irradiation presented in the UNSCEAR 2000 Report