The pharmacology of the antiestrogen tamoxifen is reviewed. The drug is currently used extensively in the treatment of all stages of breast cancer and is being considered as a preventive agent for women at high risk for breast cancer. Extensive laboratory studies demonstrate that tamoxifen is a tumoristatic agent in models of mammary carcinogenesis. Any clinical applications must therefore consider long-term (5-10 years) treatment strategies. Tamoxifen prevents rat mammary carcinogenesis. However, the timing of the carcinogenic insult is unknown among women. Tamoxifen must be considered to be a chemosuppressive agent to prevent the appearance of the primary tumor rather than to prevent the initial carcinogenic insult.