To examine sociodemographic, menstrual, reproductive, and other factors which may influence the age at natural menopause, the authors analyzed data from a large series of women participating in a nationwide breast cancer screening program conducted between 1973 and 1980. Standard life table techniques permitted assessment of factors suspected of varying the time to menstrual cessation among 983 premenopausal women, 1091 surgically menopausal women, and 1423 naturally menopausal women. The median age at natural menopause was 51.1 years. Multivariate analysis indicated that parity, irregularity of menstrual cycles before age 25 or first livebirth, and high socioeconomic status were significantly related to menopausal age. These data provide evidence for the hypothesis that certain environmental and hormonal factors which affect ovulation during reproductive years may ultimately postpone the menopause.