ATAC - Arimidex, Tamoxifen Alone or in Combination

Inclusion Criteria: * Patients with histologically proven operable invasive breast cancer * Patients who have completed all primary surgery and chemotherapy (if given), and are candidates to receive hormonal adjuvant therapy * Women defined as post-menopausal Exclusion Criteria: * Patients in whom there is any clinical evidence of metastatic disease * Patients who, for whatever reason (e.g. confusion, infirmity, alcoholism), are unlikely to comply with trial requirements * Patients whose chemotherapy was started more than 8 weeks (ie 56 days) after completion of primary surgery or whose chemotherapy was completed more than 8 weeks (ie 56 days) before starting randomised treatment