The purpose of the work was an assessment of thyroid function and determination of the development of antithyroid autoantibodies in children undergoing iodine prophylaxis after Chernobyl disaster. The level of T3 and T4 was determined by the RIA method, that of TSH by th immunoenzymatic method using Abbott kit, while antimembrane and antithyroglobulin antibodies were determined by the immunoenzymatic method using HTL-Plastomed kit. The studies were carried out in 844 children (433 boys and 411 girls) aged from 4 to 16 years, the selection of whom was described in our earlier report (12). Our studies demonstrated an increased level of T3 in 15.6% of the studied children and increased level of T4 in 12.7% of the cases. In most children the levels of hormones and TSH in the serum were normal. The antimembrane antibodies ATMA were present in 1.9% of the children (0.5% of the boys and 3.4% of the girls), while antithyroglobulin antibodies were found in 2.2% of the studied population (0.9% of the boys and 3.4% of the girls). No correlation was found between clinical picture, size of goitre, hormone level and the presence of thyroid autoantibodies. The studies indicate that radioactive contamination and iodine prophylaxis failed to cause permanent thyroid function disturbances in the studied population. The children in whom autoantibodies or abnormalities in hormone levels were found require further observation.