The Nurses' Health Study (NHS) was initiated in 1976 as a prospective cohort study, where 121,701 female registered nurses between the ages of 30 and 55 years were recruited from 11 US states. The Health Professionals Follow-up Study (HPFS) was initiated in 1986 and recruited 51,529 US men between the ages of 40-75. Every two years, participants updated their information on medical history, lifestyle, and incidence of chronic diseases using validated questionnaires. Follow-up rates are at least 90% of potential person-years of follow-up and mortality follow-up is more than 98%.