Breast cancer risk is influenced by endogenous hormones. Physical activity may offer one means for the primary prevention of breast cancer through its influence on ovarian hormones. This influence is manifested by changes in age at menarche and the number and nature of ovulatory menstrual cycles, factors that themselves are related to breast cancer risk. Animal experimental studies show that breast cancer risk is decreased by exercise at the time of tumor initiation but not necessarily by exercise during tumor promotion. Epidemiologic studies indicate, overall, a decreased risk of breast cancer among those women who are more physically active, whereas experimental studies of the effects of exercise in women have shown that exercise can influence characteristics of the menstrual cycle. Nevertheless, the experimental studies in women and epidemiologic studies of physical activity and breast cancer risk have been hampered by a number of methodologic limitations. The major problems in the epidemiologic studies include crude and incomplete measurements of physical activity over a woman's lifetime and inadequate control for potential confounding factors. Experimental studies of the effects of physical activity on menstrual activity in women have not adequately quantified the intensity of activity and the resultant effects on menstrual cycle changes.