Tamoxifen is an oral antiestrogen first used in metastatic breast cancer in the early 1970s. Large clinical trials were initiated in the late 1970s and early 1980s to test the drug's role as adjuvant therapy in early stage breast cancer. Observations of marked decreases in the development of contralateral breast cancer among tamoxifen recipients suggested potential for the drug in chemoprevention of breast cancer, and a large clinical trial to test the efficacy of tamoxifen in prevention of invasive breast cancer among women at increased risk was implemented in the United States in 1992. This paper reviews the rational for the clinical studies of tamoxifen as a chemopreventive agent for breast cancer and summarizes new information that has contributed to our understanding of tamoxifen's actions at the molecular and clinical levels. Current knowledge about the drug's mechanism of estrogenic and antiestrogenic action and its beneficial effects on blood lipids and bone metabolism will be presented. Recent research findings about DNA adduct formation and hepatic lesions, tamoxifen-associated gynecologic conditions, and the occurrence of second primary cancers in other organ systems will also be discussed.