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Understanding the BIG results: Insights from the BIG 1-98 trial analyses.Third-generation aromatase inhibitors (AIs), including letrozole, are now standard therapy for initial adjuvant endocrine treatment of postmenopausal women with early breast cancer. The International Breast Cancer Study Group's Breast International Group (BIG) 1-98 trial is examining efficacy and safety of letrozole or tamoxifen, whether used upfront or sequentially, for postmenopausal with hormone receptor-positive breast cancer. Women in the BIG 1-98 trial were randomized to either 5 years' monotherapy with tamoxifen or letrozole (trial arms A and B, respectively), or to 5 years' sequential therapy with 2 years of tamoxifen followed by 3 years of letrozole, or 2 years of letrozole followed by 3 years of tamoxifen (arms C and D, respectively). Results of the primary core analysis (PCA) of BIG 1-98, at a median follow-up of 25.8 months, showed a significant improvement in disease-free survival for patients treated with initial adjuvant letrozole relative to those on tamoxifen (P=0.003). Of interest, there was an important reduction in distant metastases, the most common and ultimately fatal type of early recurrence, for patients allocated to initial letrozole compared with initial tamoxifen. Since the PCA, there have been further analyses of BIG 1-98 with variations in the cohorts analyzed and follow-up period. The first and largest central pathology review in an adjuvant AI trial to date was also performed in the BIG 1-98 trial. In light of the impending sequence analysis, expected in late 2008, the goal of this article is to describe and summarize the breadth of information learned to date and highlight the key findings regarding the efficacy and safety of letrozole in the initial adjuvant setting.