Following up on the results of recent completed trials, several major breast cancer prevention trials are either underway or impending. In the Study of Tamoxifen and Raloxifene trial, eligible women are at least 35 years of age and postmenopausal, with either lobular carcinoma in situ or a 5-year risk of invasive breast cancer of at least 1.67%. The study will compare the ability of 5 years of tamoxifen or raloxifene to reduce the incidence of breast cancer. Subjects are randomly assigned to receive either 20 mg of tamoxifen or 60 mg of raloxifene daily. After 3 years of recruitment, 13647 women have been randomized (20.7% of those eligible). The median age of randomized women is 58 years (mean age, 58 years), and their median 5-year risk of breast cancer is 3.3% (mean 5-year risk of breast cancer, 4.0%). Hysterectomy was reported by 52.5% of the randomized women; lobular carcinoma in situ was reported by 8.4% of subjects before randomization. In the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-24 trial, 1804 women with ductal carcinoma in situ were randomly assigned tamoxifen after lumpectomy and radiation therapy. Women in the tamoxifen group had fewer breast cancer events at 5 years than did those on placebo (8.2% versus 13.4%, P = 0.0009). The proposed NSABP B-35 trial will have the same design as NSABP B-24 but will compare tamoxifen with anastrozole in postmenopausal women. Outcomes will include both ipsilateral and contralateral new breast cancer and recurrences, as well as the occurrence of regional and distant disease. Enrollment will begin in early 2003.