The objectives are (i) to identify thyroid cancer cases and demographically matched controls without thyroid cancer occurring among the 150,000 Ukrainian clean-up workers through linkage to the National Cancer Registry and Chernobyl State Registry of Ukraine; (ii) to trace/locate and interview a small sample of subjects with thyroid cancer and their respective controls ; (iii) to obtain saliva buccal cell samples; and (iv) to locate and transfer thyroid tumor specimens from identified cases to the archive maintained at the Research Center for Radiation Medicine (RCRM),for diagnostic verification and possible future genetic/molecular research using the latest available techniques.