In 1977, tamoxifen, a nonsteroidal antiestrogen, was approved in the United States for the management of advanced breast cancer in postmenopausal women. Since that time, tamoxifen's therapeutic role has grown to include management of advanced breast cancer in premenopausal women, systemic adjuvant therapy for early breast cancer in premenopausal and postmenopausal women, and treatment of breast cancer in men; the drug is now being studied in chemoprevention trials for women at high risk for breast cancer. Tamoxifen therapy prolongs disease-free survival and reduces mortality in premenopausal, postmenopausal, node-positive, and node-negative patients. Positive objective responses are more frequent in patients with estrogen receptor-positive tumors. The role of tamoxifen in combination with chemotherapy needs to be further clarified with respect to the optimal regimen, sequential vs concomitant therapy, and appropriate patient selection. While the optimum duration of tamoxifen therapy has not yet been established, tamoxifen remains a major therapeutic agent in the medical management of breast cancer.