After the first report of the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial, adjuvant aromatase inhibitor use increased rapidly among National Comprehensive Cancer Network member institutions. Increased aromatase inhibitor use was associated with older age, vascular disease, overexpression of human epidermal growth factor receptor 2 (HER2), or more advanced stage, and substantial variation was seen among institutions. This article examines adjuvant endocrine therapy in postmenopausal women after the first report of the trial, identifies temporal relationships in aromatase inhibitor use, and examines characteristics associated with choice of endocrine therapy among 4044 postmenopausal patients with hormone receptor–positive nonmetastatic breast cancer presenting from July 1997 to December 2004. Multivariable logistic regression analysis examined temporal associations and characteristics associated with aromatase inhibitor use. Time-trend analysis showed increased aromatase inhibitor and decreased tamoxifen use after release of ATAC results (P < .0001). In multivariable regression analysis, institution (P <. 0001), vascular disease (P <. 0001), age (P = .0002), stage (P = .0002), and HER2 status (P = .0009) independently predicted aromatase inhibitor use. Institutional rates of use ranged from 15% to 66%. Adjuvant aromatase inhibitor use increased after the first report of ATAC, with this increase associated with older age, vascular disease, overexpression of HER2, or more advanced stage. Substantial variation was seen among institutions.