The androgen receptor (AR) is expressed widely in breast cancer and has been proposed as a therapeutic target in patients who do not express the oestrogen receptor (ER). However, its role in women with ER-positive tumours is uncertain. The AR inhibitor, enzalutamide, exerts its effects by preventing nuclear localization of the AR. Now, a study in 192 women with ER-positive breast cancer has shown that the AR:ER ratio is a significant predictive factor for risk of breast cancer. A high AR:ER ratio demonstrated a fourfold increased risk of failure to respond to tamoxifen treatment, and the AR:ER ratio was an independent predictor of disease-free survival and disease-specific survival. In breast cancer xenograft models, enzalutamide was also shown to inhibit oestradiol-mediated cell growth, indicating that this agent could be used to treat AR-positive tumours, regardless of the ER status.