Every year, more than 2000 New Zealand women are diagnosed with oestrogen receptor positive breast cancer. Most of these women are treated with therapies that target production or action of the hormone oestrogen. However, some patients obtain little to no benefit from this treatment and their cancer returns. We have previously found that women with high levels of immune cells within their tumours have poorer outcomes. To find ways to improve the response of these tumours to therapy, we aim to trial a short treatment of the common anti-inflammatory drug aspirin together with standard anti-oestrogen therapy. We will determine whether administering these two drugs together will decrease the number of immune cells entering the tumour and the rate at which the tumour grows. This analysis will indicate whether aspirin is likely to help patients longer term and define biomarkers to identify which patients are in greatest need of additional therapies.