Purpose; To compare the efficacy of adjuvant anastrozole and letrozole in hormone receptor-positive postmenopausal patients with early breast cancer. Methods; A total of 569 hormone receptor-positive post-menopausal early breast cancer patients were included and analyzed in this study. Of them 238 were taking adjuvant anastrozole and 331 adjuvant letrozole. Demographic and medical data including age, menopausal status, weight, height, treatment history and comorbid diseases were collected from their medical charts. Results; In both anastrozole and letrozole users, the baseline clinicopathologic characteristics and the treatment history with radiotherapy and chemotherapy were similar. The median patient follow-up was 26.4 months. In the anastrozole arm disease free survival (DFS) was 94.9, 81.3 and 66.0%, whereas in the letrozole arm DFS was 90.6, 78.7 and 68.5% in the first, third and fifth years, respectively (p=0.25). Median overall survival (OS) could not be reached due to the low number of events in both arms. Three-year survival rate in the anastrozole arm was 98.8%, whereas in the letrozole arm it was 96.7% (p = 0.20). Conclusion; This study showed that both letrozole and anastrozole have similar effects on DFS and OS in the adjuvant hormonal treatment of postmenopausal hormone receptor-positive breast cancer. We believe that this retro-spective study is the first to directly compare the efficacy of letrozole and anastrozole.