After the Chernobyl accident a statistically significant increase in the number of children with thyroid tumours was observed. In this study 166 children with and 75 without thyroid tumours were analysed for micronucleus formation in peripheral blood lymphocytes using the cytochalasin B approach. The following factors did not significantly affect micronucleus formation: gender, age at the time of the first (131)I treatment, tumour stage, tumour type, or metastases; a statistically significant increase in the number of micronuclei, however, was observed for the residents of Gomel compared to other locations, such as Brest, Grodno, and Minsk. The children with tumours received (131)I treatment after surgical resection of the tumours. This gave us the opportunity to systematically follow the effect of (131)I on micronucleus formation. A marked increase was observed 5 days after the (131)I treatment followed by a decrease within a 4-7 months interval up to the next application, but the pre-treatment levels were not achieved. Up to 10 therapy cycles were followed each including an analysis of micronucleus formation before and 5 days after (131)I application. The response of the children was characterised by clear individual differences and the increase/decrease pattern of micronucleus frequencies induced by iodine-131 was correlated with a decrease/increase pattern in the number of lymphocytes.