The growth retardation following irradiation of long bones during childhood has been studied. In 9 patients, irradiation was given for treatment of hemangioma localized in the extremities. In another 13 patients, irradiation was administered because of different malignancies localized either in the long bones themselves or in the soft tissues surrounding the epiphyseal plates. The deficit in growth of the irradiated extremity could, in most of the patients, be assessed at the time when the period of growth was completed. An attempt was made to derive dose-effect curves as a function of the age of the children at the moment of irradiation and the administered irradiation dose. When the parameter used for estimating the radiation effect was the shortening of the irradiated extremity as compared to the unirradiated one, the effect was greater in younger bones. However, when the growth remaining after irradiation was taken into account the age at irradiation did not influence the final effect, the dose of irradiation being the most important factor.