NCI, together with collaborators in Belarus 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 in April 1986 and potentially exposed to radioactive iodines, principally iodine 131 (I-131). Members of the study cohort (approximately 12,000) have been given biennial screening examinations of the Thyroid and completed the third 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, residence history, and environmental models of I-131. Active screening was completed in 2008. Follow-up of the cohort continues through linkage with the Belarus Cancer Registry.