The Million Women Study is a population-based cohort study of 1.3 million women born in 1935-1950 with the aim of providing reliable information on potentially modifiable causes of common and serious illnesses. Participants’ health has been followed mainly through linkage to routinely collected NHS records as well as re-surveys. Current focus is on risk factors for cancer, dementia, osteoporosis, stroke and other severe disabling conditions that become increasingly common as women age.