Major advances in the past year in the etiology, early detection, and prevention of breast cancer are reviewed. Progress has been made in the genetic epidemiology of breast cancer. A major gene locus has been identified among cancer families with predominantly breast or ovarian cancer, or both. Hormonal factors associated with breast cancer continue to be studied. Recent reports suggested that exogenous hormone therapy is associated with only moderate or no excess risk of breast cancer. With respect to early detection, controversy arose regarding the benefit of screening for women younger than 50 years of age. The Canadian National Breast Screening Study showed no decrease in breast cancer mortality for women 40 to 49 years old who were screened annually compared with women who had a baseline breast examination. An international workshop was convened to review results of breast cancer screening trials with emphasis on women younger than 50 years of age. Chemoprevention trials for the primary prevention of breast cancer are underway in North America and Europe. Agents under investigation include tamoxifen and fenretinide. Prevention of breast cancer will also be examined in two trials with multiple disease end points; one is an intervention with low dietary fat, and the other is an intervention with specific micronutrients.