Methods Participants were postmenopausal patients with invasive operable breast cancer who had completed primary therapy and were eligible to receive adjuvant hormonal therapy. The primary endpoints were disease-free survival and occurrence of adverse events. Analysis for efficacy was by intention to treat.

Findings 9386 patients were recruited, of whom 3125 were randomly assigned anastrozole, 3116 tamoxifen, and 3125 combination. Median follow-up was 33.3 months. 7839 (84%) patients were known to be hormone-receptor-positive. Disease-free survival at 3 years was 89.4% on anastrozole and 87.4% on tamoxifen (hazard ratio 0.83 [95% CI 0.71-0.96], p=0.013). Results with the combination were not significantly different from those with tamoxifen alone (87.2%, 1.02 [0.89-1.18], p=0.8). The improvement in disease-free survival with anastrozole was seen in the subgroup of hormone-receptor-positive patients, but not the receptor-negative patients. Incidence of contralateral breast cancer was significantly lower with anastrozole than with tamoxifen (odds ratio 0.42 [0.22-0.79], p=0.007). Anastrozole was significantly better tolerated than tamoxifen with respect to endometrial cancer (p=0.02), vaginal bleeding and discharge (p<0.0001 for both), cerebrovascular events (p=0.0006), venous thromboembolic events (p=0.0006), and hot flushes (p<0.0001). Tamoxifen was significantly better tolerated than anastrozole with respect to musculoskeletal disorders and fractures (p<0.0001 for both).

Interpretation Anastrozole is an effective and well tolerated endocrine option for the treatment of postmenopausal patients with hormone-sensitive early breast cancer. Longer follow-up is required before a final benefit:risk assessment can be made.