On April 30, 2010, the U.S. Food and Drug Administration converted letrozole (Femara®; Novartis Pharmaceuticals Corporation, East Hanover, NJ) from accelerated to full approval for adjuvant and extended adjuvant (following 5 years of tamoxifen) treatment of postmenopausal women with hormone receptor-positive early breast cancer. The initial accelerated approvals of letrozole for adjuvant and extended adjuvant treatment on December 28, 2005 and October 29, 2004, respectively, were based on an analysis of the disease-free survival (DFS) outcome of patients followed for medians of 26 months and 28 months, respectively. Both trials were double-blind, multicenter studies. Both trials were unblinded early when an interim analysis showed a favorable letrozole effect on DFS. In updated intention-to-treat analyses of both trials, the risk for a DFS event was lower with letrozole than with tamoxifen (hazard ratio [HR], 0.87; 95% confidence interval [CI], 0.77-0.99; p = .03) in the adjuvant trial and was lower than with placebo (HR, 0.89; 95% CI, 0.76-1.03; p = .12) in the extended adjuvant trial. The latter analysis ignores the interim switch of 60% of placebo-treated patients to letrozole. Bone fractures and osteoporosis were reported more frequently following treatment with letrozole whereas tamoxifen was associated with a higher risk for endometrial proliferation and endometrial cancer. Myocardial infarction was more frequently reported with letrozole than with tamoxifen, but the incidence of thromboembolic events was higher with tamoxifen than with letrozole. Lipid-lowering medications were required for 25% of patients on letrozole and 16% of patients on tamoxifen.