The Nurses' Health Study (NHS) is a prospective cohort study of 121,700 female registered nurses aged 30-55 years at baseline in 1976. Mailed questionnaires were administered biennially to collect data on medical, dietary, and lifestyle factors, with a follow-up rate exceeding 90% for each 2-year cycle. Blood samples were collected from 32,826 participants between 1989 and 1990 (biospecimen collection cycle 1) and 18,717 participants between 2000 and 2001 (biospecimen collection cycle 2). Participants who provided a blood specimen were generally similar to those who did not in terms of diet and lifestyle