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The purpose of this study is to understand why thyroid cancer incidence rates are higher among Southeast Asian (SA) women living in the United States than among other United States women. A multiethnic population-based, case-control study of thyroid cancer among women ages 20-74 was conducted in the San Francisco Bay Area. Cases diagnosed between 1992 and 1998 were identified through the area's population-based cancer registry. Controls were identified using random digit dialing and matched to cases on age and ethnicity. Asian women were classified as SA (n = 214) or Northern Asian (n = 196) based on self-reported ethnicity. Relative attributable risks, by age group (<50 and 50+), were calculated to assess what proportion of the difference in incidence rates between these populations could be attributed to the prevalence of specific thyroid cancer risk factors, assuming common relative risks across ethnic groups. Among younger women, a history of goiter or thyroid nodules and lower consumption of isoflavones from soy-based foods account for 66% of the difference in incidence between SA and Northern Asian women. Among older women, these factors, along with recent migration, accounted for 95% of the difference between these groups. When comparing SA with Caucasian women, goiter/nodules and lower consumption of carotenoids explained 67% of the difference in incidence in younger women, whereas goiter/nodules and socioeconomic variables explained 81% of the difference in incidence in older women. A greater prevalence of goiter and thyroid nodules accounts for a substantial portion of the higher thyroid cancer incidence rates among SA women. Dietary patterns also contribute to the rate differences.