NCI, together with collaborators in Ukraine and Columbia University, is studying the risk of thyroid cancer and other thyroid diseases among individuals who were children or adolescents at the time of the Chernobyl nuclear power plant accident on April 26, 1986 and who were potentially exposed to radioactive iodines, principally iodine 131 (I-131). Members of the study cohort (approximately 13,000) have been given biennial screening examinations of the Thyroid and have completed the fourth and final round of screening. Estimated dose of I-131 is based on thyroid activity measurements at the time of the accident, reports of consumption of contaminated food and residence history, as well as environmental modeling of I-131. Findings on the risks of thyroid cancer, follicular adenoma, and other thyroid disorders diagnosed during the first round of screening have been published. Results from analysis of cancer incidence in cycles two to four will be published. Papers on the pathology of the tumors from the first cycle, autoimmune thyroiditis and antibodies to thyroid peroxidase elevations, and dosimetry have also been published. To achieve a better understanding of the mechanisms of I-131-induced carcinogenesis, investigators are conducting analyses of dose-dependent gene expression patterns in thyroid cancers diagnosed in the Ukrainian cohort. In addition, they are evaluating germline SNPs in selected DNA repair genes in relation to risk of thyroid cancer and common somatic mutations found in cancerous thyroid tissue. The study has transitioned to other forms of follow-up, including record linkage with the Ukraine National Cancer Registry.