 The ATENA phase III randomized parallel-group multicenter trial is designed to compare 5 years of adjuvant exemestane versus 5 years of observation in postmenopausal women with operable breast cancer who have received 5-7 years of adjuvant tamoxifen. The primary endpoint for the core protocol is disease-free survival DFS. Exemestane treatment is planned for 5 years unless disease relapse or excessive toxicity is documented, the patient refuses further treatment or any new anti-cancer therapy is initiated. 