Adjuvant endocrine therapy with tamoxifen has a clearly established benefit in postmenopausal women with resected early breast cancer that expresses the estrogen receptor and/or progesterone receptor. Whereas there is a vast and long experience with tamoxifen, the major focus of clinical trials over the past 6 years has involved the study of the third-generation aromatase inhibitors. Recently published data from the Arimidex, Tamoxifen, Alone or in Combination (ATAC) trial, which involved only postmenopausal women and is the largest adjuvant trial ever conducted, has demonstrated superior efficacy for anastrozole over tamoxifen alone or in combination with anastrozole. These data have engendered a great deal of discussion as to whether they provide a sufficient basis for changing the standard of practice in terms of choice of agent. Currently, a case can be made that further maturation of the ATAC trial is necessary and that tamoxifen should remain the standard of endocrine adjuvant therapy for most women, with anastrozole being used in those with a contraindication or intolerance to tamoxifen. However, close attention must be paid to new information as it develops not only related to efficacy but also to other end organ effects.