The Yale Cancer Center will participate in a national trial to determine the effectiveness of two drugs in preventing breast cancer. The STAR Trial, also known as the Study of Tamoxifen and Raloxifene, began this spring and will involve 22,000 post-menopausal women at increased risk for developing breast cancer. The National Surgical Adjuvant Breast and Bowel Project (NSABBP) is conducting the study.Tamoxifen, which has been in use for 20 years to treat cancer, also has been found effective in preventing breast cancer. In a previous trial the NSABBP found a 49 percent decrease in invasive breast cancer among women who received tamoxifen therapy. The new study will seek to determine whether raloxifene is also effective in preventing breast cancer and whether it results in fewer side effects than tamoxifen. Raloxifene was recently approved by the FDA to prevent osteoporosis in postmenopausal women.