Breast and thyroid cancers pose an increasing health threat to young adult women ages 20-39 years in the United States (U.S.). Analysis of cancer surveillance data for the years 1992-2011 from 13 U.S. Surveillance, Epidemiology, and End Results (SEER) Cancer Registries identified that the annual incidence of Stage I-III hormone receptor-positive breast cancer among women ages 20-29 years and 30-39 years in Iowa, ranked second and fourth, respectively among the 13 registries. A recent review of Iowa SEER Cancer Registry data identified that the incidence of thyroid cancer has increased markedly from 1974-1983 to 2004-2013 among Iowa women ages 25-39 years. Even with this changing epidemiology, risk factors that contribute to these premenopausal malignancies are not well studied. Moreover, factors examined for postmenopausal breast and thyroid cancers do not seem to explain their increased incidence among premenopausal women. Because premenopausal breast and thyroid cancers are often hormonally mediated, their onset may be influenced by exposure to endocrine disrupting chemicals (EDCs), and such exposure could begin prenatally. To date, investigation of prenatal exposures and premenopausal breast and thyroid cancers is limited, and investigation of both prenatal and postnatal exposures is largely unexplored. As such, we propose to take full advantage of the high-quality, population-based cancer surveillance data collected by the Iowa SEER Cancer Registry and the multi-decade repository of population-based birth and death data maintained by the Iowa Registry for Congenital and Inherited Disorders to examine prenatal and postnatal exposures in the etiology of breast and thyroid cancers among a cohort of young adult Iowa women. We hypothesize that breast cancer and thyroid cancer in young adult Iowa women have geospatial predilections, suggesting estrogenic environmental risks from birth to diagnosis. For breast cancer, we also hypothesize that differences in endogenous hormone exposure are positively associated with increased incidence and worse outcomes for young adult women with hormone receptor-positive than those with hormone receptor-negative disease. To test these hypotheses, we aim to: 1) link cancer surveillance data with birth and death data to establish and describe a cohort of young adult Iowa women with breast or thyroid cancer; 2) conduct individual-level geospatial surveillance for young adult Iowa women with breast or thyroid cancer; and 3) examine associations between selected environmental exposures at birth and at diagnosis and development of breast or thyroid cancer in young adult Iowa women. In conducting these aims, we will eschew more traditional study designs, and instead, implement a population-based, life course, household-level design. Our proposed design also will allow us to easily incorporate annual updates of cancer surveillance and birth and death data to monitor future patterns of cancer occurrence and emerging exposures in this cohort. Elucidating etiologies of breast and thyroid cancers in young adult women is critical due to the prevalence of these diseases, as well as the marked personal and societal impacts of cancer diagnosis at a young age. Our study can help provide insights into these etiologies, an important step to developing interventions to reduce the incidence of these malignancies in young adult women.