Thyroid cancer is known to be caused by exposure to ionizing radiation, but the radiation risk associated with exposure in adult males is not well characterized. We are planning to conduct a case-control study nested in the population of 150,000 male liquidators who participated in the clean-up operation following the Chornobyl accident. In preparation of this study, we are conducting a pilot study of thyroid cancer cases and matched controls in this liquidator cohort. Thyroid cases identified from the Ukraine Cancer Registry are located and interviewed for information needed to estimate thyroid doses and for other factors suspected to be associated with thyroid cancer. Study participants are asked to provide biological materials for future genetic studies. We also obtain information on whereabouts of thyroid tumor tissue using various databases.