A total of 668 patients (340 anastrozole and 328 tamoxifen) were randomised in a double-blind, double-dummy multicentre study. Anastrozole was given in a dose of 1 mg once daily and compared with tamoxifen 20 mg daily in postmenopausal patients with tumours that were hormone-receptor positive or of unknown receptor status. The efficacy and tolerability of anastrozole was compared with that of tamoxifen as first-line therapy for advanced breast cancer. The median time to progression was similar for both treatments (8.2 months in anastrozole patients and 8.3 months in tamoxifen patients). Anastrozole was also as effective as tamoxifen in terms of objective response-rate with 33% in the anastrozole group and 32.6% in the tamoxifen group achieving a complete or partial response. Both treatments were well tolerated. However, incidences of thromboembolic events and vaginal bleeding were reported in fewer patients treated with anastrozole than with tamoxifen. In conclusion, these findings indicate that anastrozole can be considered as first-line therapy for postmenopausal women with advanced breast cancer.