The Million Women Study started recruiting participants 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. 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.

The study includes 1 in 4 of all UK women born between 1935 and 1950, recruited through NHS breast screening centres in England and Scotland in 1996-2001. The 66 NHS breast screening centres that recruited participants covered about half of the UK population.

While the initial stimulus was to study the risk of breast cancer and other conditions associated with the use of menopausal hormone therapy, most of the women who joined the study had reached adulthood in the 1960s and had considerably different lifestyles to those of previous generations. For example, large proportions had begun smoking and using oral contraceptives as teenagers and young adults. The prevalence of obesity was also increasing and there were claims that other lifestyle factors also had important effects on health. To answer questions about the effects of these factors on health reliably requires large scale population-based evidence. The Million Women Study is therefore investigating the short-term and long-term effects of these and many other factors on women’s risk of developing or dying from different types of cancer, heart disease, stroke, dementia and other mental and neuro-degenerative disorders, and many other conditions in middle and in old age.

For further details on the study design, recruitment, data collection and other aspects of the Million Women Study, please visit https://www.ceu.ox.ac.uk/research/the-million-women-study