PIP: The various risk factors for breast cancer have been recognized for many years. A table lists these established breast cancer risk factors together with the approximate magnitude of the increase in risk associated with them. Breast cancer incidence rates increase with age throughout the life span in Western countries, although the rate of increase is greater up to age 50 years than after 50 years. Breast cancer is more common among women in upper rather than lower social classes, among women who never have been married, among women living in urban areas, among women living in the northern US than in the southern US, and among whites than blacks, at least among those over age 50. Women in North American and Northern European countries have the highest risk for breast cancer, women in Southern European and Latin American countries are at intermediate risk, and women in Africa and Asian countries have the lowest risk. Yet, rapid rates of increase in incident rates have been noted in recent years in many Asian, Central European, and some South American countries. The later the age at which a woman has her 1st full-term pregnancy, the higher her risk for breast cancer; the earlier the age at menarche and the later the age at menopause the higher the risk; and among women who have a premenopausal oophorectomy, the earlier the age at which this occurs the lower the risk. Among postmenopausal women, obesity is associated with an increase in risk. Lactation is negatively associated with subsequent breast cancer risk. Some current research is considering potential risk factors that have not been well studied in the past, including alcohol consumption, cigarette smoking, caffeine consumption, exposure to diethylstilbestrol (DES), emotional stress, exposure to electric power, and lack of physical activity. Other areas of current research reviewed here include radiation, mammographic parenchymal patterns, a high-fat diet, use of oral contraceptives (OCs), use of estrogen replacement therapy, and endogenous hormones. Cigarette smoking and caffeine consumption do not appear promising as potential etiologic agents. The studies of the DES-exposed women and of OC users suggest that the timing of exposure may be critical, since the possible effect of both these hormonal agents may be limited to specific time periods of rapid breast development. If such a critical period does not exist in postmenopausal women, then there may be little effect of hormones used at this time. Studies with long-term follow-up and that include long-term users are essential to studies of effects of hormones and other exposures.