Letrozole, a third-generation aromatase inhibitor, has been the only aromatase inhibitor to date to show unequivocal superiority to tamoxifen as first-line treatment of metastatic postmenopausal breast cancer. The superiority of letrozole compared with tamoxifen was also reflected in the neoadjuvant setting, in both estrogen receptor-positive and estrogen receptor-unknown patients with differing HER-2 status. Currently, studies are being performed in the adjuvant setting, which will provide important data on the long-term safety of letrozole and help determine its suitability as a chemopreventive agent in healthy women at risk of developing breast cancer. Nevertheless, the superior clinical efficacy and survival data of letrozole suggest that it has the potential to displace tamoxifen as the gold standard in breast cancer treatment in the coming years.