BACKGROUND: The long-term effects of ionising radiation, including radioiodine, on thyroid function are not well known. We compared thyroid immunity and function in two groups of children from Belarus, one of whom was exposed to the radioactive fallout of Chernobyl. METHODS: We measured serum free thyroxine 4 (free T4), free T3, and thyrotropin hormone (TSH) and the prevalence of thyroid autoantibodies (antithyroglobulin and antithyroperoxidase), in 287 children or adolescents living in Hoiniki (average caesium contamination of 5.4 Ci/km2). We also studied 208 children and adolescents living in Braslav (average contamination <0.1 Ci/km2), who were age 12 years or less at the time of the Chernobyl accident. FINDINGS: The prevalence of antithyroglobulin or antithyroperoxidase, or both, was significantly higher (p=0.0001) in individuals living in Hoiniki (56 [19.5%] of 287) than in those living in Braslav (eight [3.8%] of 208). In both villages, no sex differences were found in the antibody prevalence before age 13 years. Thereafter, a significantly higher prevalence of thyroid autoantibodies was found in girls from Hoiniki. The increase in the prevalence of circulating antibodies in the contaminated group was already apparent in individuals who, at the time of the accident, were in utero or newborn (15.7%), and was even more pronounced in children of 9 years or more (35.1%). No major alterations of serum FT-4, FT-3, or TSH were found. INTERPRETATION: 6-8 years after the Chernobyl accident, a significant increase in thyroid autoimmunity was found in children exposed to radioactive fallout. Pubertal age in girls is a risk factor for increased prevalence of thyroid autoimmunity. The autoimmune phenomena are limited to an increased prevalence of circulating thyroid autoantibodies without evidence of significant thyroid dysfunction. The future development of clinically relevant thyroid autoimmune diseases, especially hypothyroidism, is a possibility.