Selective aromatase inhibitors cause regression of breast carcinomas by reducing estrogen production via inhibition of the enzyme aromatase (estrogen synthetase). A higher incidence of hormone-dependent breast cancer occurs in postmenopausal women than in younger women. Thus, total estrogen blockade is more likely to be achieved with systemic (pharmacologic) methods than by surgical removal of endocrine glands. At the present time, breast cancer patients with hormone-dependent disease usually receive the antiestrogen tamoxifen as first-line treatment. Although tamoxifen is effective initially, patients develop resistance to the drug, which results in disease progression. Aromatase inhibitors acting by a different mechanism from tamoxifen are effective in some of these patients. There is also the potential that aromatase inhibitors could be more effective as first-line treatment. Formestane, 4-hydroxyandrostenedione, the first approved aromatase inhibitor, is proving to be useful in postmenopausal breast cancer patients with advanced disease. Recently, anastrozole, a nonsteroidal agent, has been approved in the United States. Other very potent aromatase inhibitors are under development. Aromatase inhibitors as a new class of well-tolerated agents are now becoming available for improving the treatment of breast cancer patients.