The Nurses’ Health Study is a longitudinal population study aimed initially to examine the relation between the use of oral contraceptives, cigarette smoking, and risk of major illnesses in women, mainly cancer and cardiovascular diseases. Since then, the study broadened to include the evaluation of health consequences of many lifestyle practices, including diet, physical activity, and specific forms of hormone therapy.

The participants are registered nurses, aged 30 to 55 years and married at the time of recruitment in 1976, and who lived in the 11 most populous states (California, Connecticut, Florida, Maryland, Massachusetts, Michigan, New Jersey, New York, Ohio, Pennsylvania and Texas).