Female breast cancer is a major medical problem with significant public health ramifications. During the next decade, more than 1.5 million women in the United States will be newly diagnosed with invasive breast cancer. The Breast Cancer Prevention Trial (BCPT) is an NIH-sponsored study which will determine whether long-term tamoxifen therapy is effective in: 1) Preventing invasive breast cancer, 2) Lowering the incidence of fatal and nonfatal myocardial infarction, and 3) Reducing the incidence of bone fractures. Sixteen thousand women, including at least 100 from West Virginia who are > or = 35 years of age at increased risk for breast cancer, will be randomized between placebo and tamoxifen during a two-year period. For each participant, the annual and lifetime probability of developing breast cancer will be estimated utilizing a computerized model of risk assessment. The placebo or tamoxifen will be administered for at least five years. Toxicity, compliance, monitoring of quality-of-life assessment, and lipid and lipoprotein evaluations are major components of this trial.