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Background Current use of hormone-replacement therapy (HRT) increases the incidence of breast cancer. The Million Women Study was set up to investigate the effects of specific types of HRT on incident and fatal breast cancer. Methods 1 084 110 UK women aged 50–64 years were recruited into the Million Women Study between 1996 and 2001, provided information about their use of HRT and other personal details, and were followed up for cancer incidence and death. Findings Half the women had used HRT; 9364 incident invasive breast cancers and 637 breast cancer deaths were registered after an average of 2·6 and 4·1 years of follow-up, respectively. Current users of HRT at recruitment were more likely than never users to develop breast cancer (adjusted relative risk 1·66 [95 % CI 1·58–1·75], p<0·0001) and di