Chromosome aberrations induced in vivo were studied in nine children 5-12 years old treated with total-body high-energy photon irradiation (pulsed exposure from a LINAC) for different types of malignant diseases. Dose-effect relationships were obtained for each child by taking blood at different times during exposure. In vitro dose-effect relationships for chromosome aberrations in children and adults were obtained by exposing blood under the same conditions as the children. Exposure in vivo and in vitro yielded similar linear-quadratic dose-effect relationships for dicentric aberrations. The response in vitro was slightly greater than in vivo, but the difference was not very large. It is concluded that the dose-effect relationship for dicentric chromosome aberrations obtained in vitro for adults can be used for biological dosimetry in irradiated children. Some of the children displayed a high number of "rogue cells" before exposure; this may be due to the malignant disease as it was not found in the healthy controls.