Background. The waiting period to surgery represents a valuable “window of opportunity” to evaluate novel therapeutic strategies. Interventional studies performed during this period require significant multidisciplinary collaboration to overcome logistical hurdles. We undertook a one-year prospective window of opportunity study to assess feasibility.Methods. Eligible newly diagnosed postmenopausal, estrogen receptor positive breast cancer patients awaiting primary surgery received anastrozole daily until surgery. Feasibility was assessed by (a) the proportion of patients who consented and (b) completed the study. Comparison of pre- and poststudy Ki67 labelling index and cleaved caspase 3 scores (CC3) was performed.Results. 22/131 (16.8%) patients were confirmed eligible and 20/22 (91%) patients completed the study. 19/20 (95%) patients agreed to undergo optional additional tissue biopsies. The mean duration of anastrozole use was 24.7 (15–44) days. There were a statistically significant decline in mean Ki67 indices of 48.8% () and a trend towards significance in the decline of CC3 () when comparing pre- with posttreatment values.Conclusion. window of opportunity trials in breast cancer are a feasible way of assessing the biologic efficacy of different therapies in the presurgical setting. The majority of eligible women were willing to participate including undergoing additional tissue biopsies.