Endometrial cancer and hormone-replacement therapy in the Million Women Study

Postmenopausal women who use hormone-replacement therapy (HRT) containing oestrogen alone are at increased risk of endometrial cancer. To minimise this risk, many HRT users who have not had a hysterectomy use combined oestrogen-progestagen preparations or tibolone. Limited information is available on the incidence of endometrial cancer in users of these therapies. 716 738 postmenopausal women in the UK without previous cancer or previous hysterectomy were recruited into the Million Women Study in 1996–2001, provided information about their use of HRT and other personal details, and were followed up for an average of 3·4 years, during which time 1320 incident endometrial cancers were diagnosed. 320 953 women (45%) reported at recruitment that they had used HRT, among whom 69 577 (22%) last used continuous combined therapy (progestagen added daily to oestrogen), 145 486 (45%) last used cyclic combined therapy (progestagen added to oestrogen, usually for 10–14 days per month), 28 028 (9%) last used tibolone, and 14 204 (4%) last used oestrogen-only HRT. These HRT types had sharply contrasting effects on the overall risk of endometrial cancer (p