The mechanism of action, pharmacology, and clinical efficacy and safety of exemestane in the treatment of metastatic breast cancer are reviewed. Endocrine strategies that deprive tumor cells of estrogens are effective therapeutic modalities for patients with hormone-dependent breast cancer. The efficacy and toxicities associated with tamoxifen and aminoglutethimide have contributed to the development of agents that selectively target aromatase, the enzyme responsible for conversion of androgens to estrogens. Exemestane, an orally active irreversible inhibitor of aromatase, was recently approved as second-line endocrine therapy of advanced hormone-sensitive postmenopausal breast cancer. Compared with megestrol acetate in patients with disease progressing on tamoxifen, a number of clinical endpoints, including a survival advantage, were significantly better in the exemestane-treated group. Preliminary data also indicate that cross-resistance is incomplete between exemestane and the reversible aromatase inhibitors. Even though suppression of aromatase activity is quantitatively similar regardless of the interactive mechanism between drug and enzyme, clinically relevant differences may become apparent with further application of these two types of aromatase inhibitors. Exemestane is effective as a second- or third-line treatment of advanced, estrogen-receptor positive breast cancer in postmenopausal patients.