Tamoxifen reduced the risk of invasive breast cancer by 49% among women at increased risk for breast cancer in the Breast Cancer Prevention Trial P-1, and raloxifene reduced breast cancer incidence by more than 70% in the Multiple Outcomes of Raloxifene Evaluation osteoporosis trial. These findings led the National Surgical Adjuvant Breast and Bowel Project to design and launch the Study of Tamoxifen and Raloxifene. Risk-eligible women are = 35 years of age and postmenopausal; they have either lobular carcinoma in situ (LCIS) or a 5-year risk of invasive breast cancer of at least 1.67% as determined by the Gail model. Participants are randomly assigned to receive either tamoxifen 20 mg or raloxifene 60 mg daily. The trial opened for accrual on July 1, 1999. After 32 months of recruitment at 194 clinical centers in North America, risk assessments have been performed in 107,855 women (83.8% white, 9.4% black, 3.8% Hispanic, 3.1% other race/ethnic groups). Of the eligible patients, 12,637 have been randomized (20.9% of risk-eligible women); the median age is 58 years (mean, 58 years), and the median 5-year risk of breast cancer is 3.3% (mean, 4.0%). LCIS was reported in 8.4% of women prior to randomization. Gail model risk was = 3.0% in 5 years for 59.3% of white women, 45.0% of black women, and 44.5% of Hispanic women. The trial will recruit a total of 22,000 postmenopausal women and is powered to demonstrate superior efficacy of either agent or their equivalence in reducing the incidence of primary breast