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 found that women with high levels of inflammatory immune cells within their tumours have poorer responses to anti-oestrogen therapy. The common anti-inflammatory drug aspirin has previously been shown to be associated with improved breast cancer outcomes in patients taking aspirin for other reasons. We hypothesise that the addition of aspirin to anti-oestrogen therapy will improve response to anti-oestrogen therapy. We plan to examine the molecular changes that occur during treatment with aspirin and anti-oestrogen therapy. This analysis will indicate whether aspirin is likely to help patients in the long term and define biomarkers to identify which patients will benefit most.