The nuclear power plant accident at Chornobyl released large quantities of Iodine-I31 into the atmosphere, contaminating thousands of square kilometers and exposing millions of people. For this study, a well-defined cohort of approximately 12,000 Belarusian children aged 0-17 at the time of the accident is being examined for thyroid disease by well-trained specialists every two years for a minimum of three screening cycles. The study is a collaborative effort of researchers in Belarus and the United States. Under a standardized protocol subjects have received three biennial thyroid examinations, including palpation, ultrasound scanning, and thyroid hormone and other laboratory tests. Information relevant for dose reconstruction is collected at the interview. Follow-up continues through linkage with the National Cancer Registry. Work is undeway to assemble a cohort of about 3,000 subjects who were in utero ar the time of the accident.