The incidence of hereditary diseases in man caused by exposure to ionizing radiation is discussed. To estimate the risk of hereditary damage from ionizing radiation, the genetically determined diseases to be considered are those whose frequency is closely linked to the mutation rate as estimated using the incidence in live-births. The concept that a dose of radiation might double the natural mutation frequency might be applied to such disorders. If for chronic irradiations with X- or .gamma.-rays the doubling dose is 100 rad, 140 cases of these hereditary diseases would occur in 1 million people of prereproductive age exposed to 1 rad.