Cohort Profile: the Million Women Study

The Million Women Study started recruiting participants over 20 years ago, in 1996. The initial stimulus was to obtain robust prospective information on the risk of breast cancer associated with use of different types of menopausal hormone therapy (HT). When planning the necessary large-scale prospective study, an equally important aim was to obtain reliable information on the effects of other potentially modifiable factors that affect women’s health as they age.