This population-based epidemiologic study will examine the role of nutritional factors and migration in thyroid cancer risk among women, with special emphasis on Asian Americans, a high risk population. Thyroid cancer is one of the most commonly occurring cancers among young women and in the rapidly growing population of Asian Americans. The higher incidence of thyroid cancer among Asian immigrants to the United States than among native Asian populations, suggests that readily changeable environmental exposures, such as nutritional factors, acting at a late stage in the carcinogenic process, affect thyroid cancer risk. Limited ecologic, experimental, and epidemiologic evidence suggests that high iodine exposure may increase the risk of papillary thyroid cancer, the most common histologic type in the United States. Alternatively, some of the findings attributed to high levels of iodine exposure may be explained by low selenium consumption. Increased consumption of other antioxidants may also play a role in decreasing thyroid-cancer risk. The proposed case-control study, designed to clarify these associations, will be conducted in the San Francisco Bay Area, an ethnically and culturally heterogenous population providing substantial variation in dietary intake. Women diagnosed with thyroid cancer will be prospectively identified during a three-year period. In addition, to test hypotheses related to migration, Asian cases diagnosed during the preceding three years will be retrospectively identified. Controls will be a probability sample of women selected from the general population using random-digit dialing, frequency-matched to cases on the basis of five-year age-groups and race/ethnicity (i.e., non-Hispanic White, African American, Hispanic/Latina, Asian American). All subjects will be between the ages of 20 and 74 at diagnosis and reside in five northern California counties covered by two population-based cancer registries. Personal interviews will be conducted with subjects in English, Spanish, Cantonese, Tagalog, or Vietnamese. Since soil levels of iodine and selenium vary widely and strongly influence food levels, intake of these trace elements derived from food-frequency questionnaires are subject to substantial misclassification. Therefore, toenail clippings, which provide a relatively long-term biologic marker of these trace elements correlated with exposure during the promotional phase of carcinogenesis, will be collected and analyzed using neutron activation analysis. Review of tumor specimens will be undertaken for cases to verify malignancy and provide a uniform histologic classification. After initial examination of the data, unconditional logistic regression analyses, adjusting for age and race/ethnicity, will be conducted to test nutritional hypotheses. Hypotheses regarding migration will be tested among Asian Americans.