Aromatase inhibitors have been shown to be superior to Tamoxifen in several settings. It is unclear whether this superiority extends to their use as primary endocrine therapy in elderly patients with early operable primary breast cancer. Biological characteristics of the tumours may aid in selecting the most suitable agent. Primary endocrine therapy with Anastrozole in 64 women >70 years with oestrogen receptor alpha-positive (ERalpha+) breast cancer was compared to that in 84 treated with Tamoxifen during the same period. Biomarkers were assessed by immunohistochemistry on diagnostic core biopsies. There was no significant difference between the two groups (Anastrozole vs Tamoxifen) in terms of clinical benefit rates at 6 months (97% vs 100%) or median progression free survival (16.5 vs 22.5 months). There were no withdrawals due to adverse events from Anastrozole, compared to four with Tamoxifen. 46%, 99%, 8% and 5% of all patients were positive for progesterone receptor, ERbeta2, HER2 and EGFR, respectively, and 64% of patients had a moderate Ki-67 index. Positive HER2 status (18 vs 21 months, p=0.003) and moderate Ki-67 index (17.5 vs 23 months, p=0.042) were associated with significantly shorter progression free survival. Results thus far show that primary endocrine therapy with Anastrozole in elderly patients with early operable ERalpha+ breast cancer is similar to Tamoxifen in terms of efficacy, but appears to be associated with less adverse events leading to withdrawals. In this population, ERalpha+ breast cancer also appears to have a less aggressive biological profile favouring better hormone sensitivity.