The authors prospectively studied the effect of demographic, reproductive, stress-related, and health behavior factors measured at study entry on age of natural menopause in 185 healthy US women. At study entry, women were 42.5-47.5 years old and premenopausal. After a baseline examination (1983-1985), women were followed for 7-9 years, during which time they reported on a monthly basis their menstrual status and whether they were taking hormones. Menopausal age was defined as age at the last menstrual period prior to stopping menstruation for 12 months (and not taking hormones). Estimated median age at menopause was 51.5 years for the whole sample. Median age at menopause was earlier for women who reported irregular menstrual cycles at study entry (50.2 years), were African-American (49.3 years), were smokers (50.6 years), or were currently on a weight reduction diet (50.5 years). Psychosocial stress was predictive of an even earlier median age at menopause in African Americans (48.4 years) and in those with irregular cycles at baseline (49.4 years). Results suggest that premenopausal women in their forties who are experiencing irregular menstrual cycles, are smokers, are dieting, or are African-American are likely to reach menopause earlier than their contemporaries. African-American women may have a different "biological clock" than white women, especially when under stress, or they may experience more stress of longer duration.