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Exemestane (Aromasin®) is an irreversible steroidal aromatase inactivator that suppresses estrogen levels, leading to regression of hormone-dependent breast cancers in postmenopausal women. It is indicated for the adjuvant treatment of estrogen receptor-positive early breast cancer in postmenopausal women who have already received 2–3 years of adjuvant tamoxifen, to complete a total of 5 years’ endocrine therapy.In the IES (Intergroup Exemestane Study) in postmenopausal women with early estrogen receptor-positive breast cancer, switching to exemestane 25 mg/day for 2–3 years after receiving tamoxifen for 2–3 years (to a total of 5 years) improved disease-free survival and reduced the risk of contralateral breast cancer compared with continued tamoxifen. Overall survival was also significantly improved in the estrogen receptor-positive/unknown population of exemestane recipients when adjusted for nodal status and prior chemotherapy and hormone replacement therapy (HRT) use. Exemestane appears to have a favorable tolerability profile compared with tamoxifen, although possible detrimental effects of estrogen suppression on bone mineral density (BMD) and cardiovascular (CV) risk require further investigation.Because of a lack of comparative trials with aromatase inhibitors, the optimal combination choices and duration of aromatase inhibitor/inactivator and tamoxifen treatment have yet to be determined; however, postmenopausal women who have already received 2–3 years of adjuvant tamoxifen for early breast cancer should be considered for switching to exemestane to complete 5 years’ endocrine therapy.