An unprecedented number of women will experience menopause in the next decade. Although the timing of menopause affects long-term disease risk, little is known about factors that affect this timing. In the present 1995--1997 cross-sectional study, the Study of Women's Health Across the Nation, the relation of demographic and lifestyle factors to age at natural menopause was examined in seven US centers and five racial/ethnic groups. All characteristics were self-reported by women aged 40--55 years (n = 14,620). Cox proportional hazards models were used to estimate the probability of menopause by age. Overall, median age at natural menopause was 51.4 years, after adjustment for smoking, education, marital status, history of heart disease, parity, race/ethnicity, employment, and prior use of oral contraceptives. Current smoking, lower educational attainment, being separated/widowed/divorced, nonemployment, and history of heart disease were all independently associated with earlier natural menopause, while parity, prior use of oral contraceptives, and Japanese race/ethnicity were associated with later age at natural menopause. This sample is one of the largest and most diverse ever studied, and comprehensive statistical methods were used to assess factors associated with age at natural menopause. Thus, this study provides important insights into this determinant of long-term disease risk in women.