Tamoxifen (ICI46,474) is a competitive inhibitor of estrogen action which has found ubiquitous application in the treatment of breast cancer. The drug is the front line endocrine therapy for breast cancer and is the proven treatment of choice for the adjuvant therapy of postmenopausal women with node-positive disease. Tamoxifen is available for the treatment of premenopausal patients with advanced disease, and is being evaluated in clinical trials as an adjuvant therapy for premenopausal patients with either node-positive or node-negative disease. Laboratory studies demonstrate that tamoxifen is a tumoristatic agent and long-term treatment strategies (chemosuppression) should be considered to apply the antiestrogen to its maximal therapeutic advantage. Optimal therapy with tamoxifen may also be achieved by treatment strategies to lower circulating estrogen levels in the premenopausal patient. Tamoxifen is a well tolerated drug, and long-term therapy does not appear to induce metabolic tolerance. Concerns about premature osteoporosis or cardiovascular disease appear to be unfounded because tamoxifen has an appropriate level of target site-directed estrogenic activity. Isolated reports about the growth or appearance of endometrial carcinoma during long-term adjuvant tamoxifen therapy must be balanced against the risks of withholding treatment to patients with a fatal disease.