Objective — to investigate the influence of in utero irradiation due to incorporated technogenic iodine-131 on the incidence of thyroid diseases in prepubescent children living under conditions of moderate iodine deficiency in the southwestern areas of the Kaluga Region. Material and methods. A total of 420 in utero irradiated children (a study group) and 150 non-irradiated children living in the same area (a control group) were followed up by a cohort method within the first 12 years of life. The incidence of thyroid diseases was estimated using the incidence rate. Results. Throughout the follow-up, the incidence rate of thyroid diseases in the in utero irradiated children was 2.5 times higher than that in the non-irradiated ones; and the morbidity level after in utero irradiation depended on the gestational period at which irradiation had happened; in this case, the highest morbidity was in the children exposed to iodine-131 at 11—13 and 16—22 weeks’ gestation. Conclusion. Thus, in utero irradiation due to incorporated technogenic iodine-131 in the presence of iodine deficiency substantially increases the incidence of thyroid diseases in prepubescent children.