Introduction

The risk of breast cancer arises from a combination of genetic susceptibility and environmental factors. Recent studies show that type and duration of use of hormone replacement therapy affect a women's risk of developing breast cancer.1^–7 The women's health initiative trial was stopped early because of excess adverse cardiovascular events and invasive breast cancer with oestrogen and progestogen.6 The publicity increased public awareness of the risks of hormone replacement therapy, and this was heightened by the publication of the million women study.2 However, the recently published oestrogen only arm of the women's health initiative trial suggests that this formulation may reduce the risk of breast cancer.8 To help make sense of the often confusing information,9 women and clinicians need individual rather than population risk data. We have produced estimates that can be used to calculate individual risk for women living up to the age of 79 and suggest the risk may be lower than is often thought.

Importance of individual data

Fears about the risks of hormone replacement therapy have resulted in reduced use.10^–12 Without individual risk data, however, it is difficult to weigh the benefits and harms of treatment accurately, and many women may have stopped treatment unnecessarily.

Although data on the lifetime risk of breast cancer (from birth to average life expectancy) are available, these are of limited value in the clinical context. This is because cumulative absolute risk declines as years of remaining life diminish, even though the age specific risk increases.13 The influence of hormone replacement therapy and other factors on absolute risk may be less in an elderly population because the number of …