Medical exposures accompanied by an increase in radiation use in the field of pediatrics were described. Basic ideas and countermeasures to radiation injuries were outlined. In order to decrease the medical exposure, it is necessary for the doctor, x-ray technician and manufacturer to work together. The mechanism and characteristics of radio carcinogenesis were also mentioned. Particularly, the following two points were described: 1) How many years does it take before carcinogenesis appears as a result of radiation exposure in infancy 2) How and when does the effect of fetus exposure appear. Radiosensitivity in infants and fetuses is greater than that of an adult. The occurrence of leukemia caused by prenatal exposure was reviewed. The relation between irradiation for therapy and morbidity of thyroid cancer was mentioned. Finally, precautions necessary for infants, pregnant women and nursing mothers when using radioisotopes were mentioned.