NCI is planning to conduct a case-control study nested in the population of male liquidators who participated in the clean-up operation following the Chernobyl accident. In preparation of this study, investigators are conducting a pilot study of 30-40 thyroid cancer cases in this liquidator cohort. Thyroid cases identified from the Ukraine Cancer Registry will be located and interviewed for information needed to estimate thyroid doses and for other factors suspected to be associated with thyroid cancer. Study participants will be asked to provide biological materials for future genetic studies. Investigators will also obtain information on whereabouts of thyroid tumor tissue using various databases.