Exemestane is an irreversible steroidal aromatase inhibitor, structurally similar to the natural substance androstenedione. In postmenopausal women, estrogens are produced primarily by the conversion of androgens to estrogens by the aromatase enzyme in peripheral tissues. Blocking estrogen production by inhibiting aromatase is an effective and selective treatment for hormone-dependent breast cancer in postmenopausal women. In postmenopausal women, exemestane significantly reduced serum estrogen concentrations starting at a dose of 5 mg; the maximum reduction (> 90%) was achieved at a dose of 10–25 mg. In postmenopausal patients with breast cancer who received 25 mg of the drug daily, total aromatase levels were reduced by 98%.