The Million Women Study is a study of women’s health analysing data from more than one million women aged 50 and over, led by Dame Valerie Beral and a team of researchers at the Cancer Epidemiology Unit, University of Oxford. It is a collaborative project between Cancer Research UK and the National Health Service (NHS), with additional funding from the Medical Research Council (UK).

One key focus of the study relates to the effects of hormone replacement therapy use on women's health. The study has confirmed the findings in the Women's Health Initiative (WHI) that women currently using HRT are more likely to develop breast cancer than those who are not using HRT.

Results from the Million Women Study, together with those of the WHI trial from the USA, have influenced national policy, including recent recommendations on the prescribing and use of hormone replacement therapy from the Royal College of Obstetricians and Gynaecologists and from the Commission on Human Medicines.

The Million Women Study is a multi-centre, population-based prospective cohort study of women aged 50 and over invited to routine breast cancer screening in the UK. Between 1996 and 2001, women were invited to join the Million Women Study when they received their invitation to attend breast screening at one of 66 participating NHS Breast Screening Centres in the UK. At these centres, women received a study questionnaire with their invitation, which they were asked to complete and return at the time of screening. Around 70% of those attending the programme returned questionnaires and agreed to take part in the study, over 1 in 4 women in the UK in the target age group. The Million Women Study is the largest study of its kind in the world.

The Million Women Study was set up with the aim of recruiting 1,000,000 women in the UK into a cohort study, to provide answers to the following questions:

Initial analysis of the results from over 1 million women in the Million Women Study appeared to confirm preliminary findings from other studies at the turn of the century finding that women currently using progestin-estrogen HRT were more likely to develop breast cancer than those who are not using HRT. This initial analysis received extensive press coverage. The initial reading of results showed that this effect is substantially greater for combined (estrogen-progestogen) HRT than for estrogen-only HRT; and that the effects were similar for all specific types and doses of estrogen and progestogen, for oral, transdermal and implanted HRT, and for continuous and sequential patterns of use. Current users of estrogen-progestogen HRT were at 2 fold increased risk of developing breast cancer, and current users of estrogen-only HRT appeared to have a 1.3 fold risk. Use of HRT by women aged 50–64 in the UK in the decade from 1993-2003 was estimated to have caused 20,000 extra breast cancers. Past users were not seen as having increased risk.

A reanalysis refuting the initial conclusion that HRT increased the risk of breast cancer, was published in a 2012 paper in the Journal of Family Planning and Reproductive Health Care. The paper's authors, led by Samuel Shapiro of the University of Cape Town, claimed that the study had not in fact establish a causal relationship between HRT and breast cancer, and that the original analysis had been flawed.

It is well known that post-menopausal women who have not had a hysterectomy are at increased risk of cancer of the endometrium (the lining of the womb) if they take estrogen-only HRT. Follow up of over 700 000 women in the Million Women Study confirmed this and showed that the risk of endometrial cancer is also increased in women who take tibolone; but is not altered, or may even be reduced, in women taking combined estrogen-progestogen HRT. These effects depend also on a woman’s body mass index (BMI, a measure of obesity) such that adverse effects of tibolone and estrogen-only HRT are greatest in thinner women, and the beneficial effects of combined HRT are greatest in fatter women.