Both oral contraceptives (OCs) and hormone replacement therapy (HRT) are widely used by healthy women, so that any increase in breast cancer has to be considered in relation to the avoidance of either pregnancy or menopausal symptoms. Use of OCs for more than four years before first full term pregnancy leads to a small but significant increase in risk of breast cancer (6.5 extra cases per 10,000 women aged 20-29) that persists for 10 years after cessation. Risk is not amplified by a positive family history and OC-induced breast cancers are more likely to be localised to the breast. For those taking HRT there is an increased relative risk of breast cancer of 2.3% per year, similar to the increase in risk for each extra year spent in a premenopausal state. The increase in risk disappears five years after cessation. HRT does not lead to any increase in risk of dying of breast cancer. Some breast cancer patients can safely take HRT, and it is possible that some formulations may reduce the subsequent risk of relapse.