Radiation histories were compiled on 251 leukemic children and compared with an equal number of matched controls. In general, exposure to radiation was significantly more frequent among the leukemic group. Thymic irradiation was found to be associated with six times the usual incidence of leukemia, the data being highly significant (p <.01). Statistical analysis of the data relative to fluoroscopic studies of the heart or gastrointestinal system, even though not highly significant, does indicate the possibility of a relationship between this type of radiation and an increased incidence of leukemia (p <.05). Diagnostic roentgenograms had been performed more frequently in the leukemic group, but the level of significance was not great (p <.05). Maternal exposure to roentgen rays during pregnancy had occurred somewhat more commonly in the leukemic group, but the difference was too small to be significant (p <.10).