The authors investigated secular trends in age at menarche, age at menopause, and reproductive life span within a population-based cohort of US women. Study subjects were 22,774 women selected randomly as controls for a case-control study. Eligible controls were residents of Wisconsin, Massachusetts, or New Hampshire born between 1910 and 1969. Subjects completed telephone interviews in 1988-2001 and answered questions regarding reproductive and lifestyle factors. Birth cohorts were created using 5- and 10-year periods, and statistical comparisons were performed with analysis of variance. The mean age at menarche decreased by approximately 6 months for those born between 1910 and 1949 (13.1 vs. 12.7 years; p < 0.001), with a subsequent increase to 13.0 years among women born between 1960 and 1969 (p < 0.001). Among naturally menopausal women aged 60 or more years who reported never use of postmenopausal hormone therapy, the authors observed a 17-month increase in the mean age at menopause for those born between 1915 and 1939 (49.1 vs. 50.5 years; p = 0.001) after adjustment for potential confounders. They also observed an increase in the average number of reproductive years (subtracting age at menarche from age at natural menopause), from 36.1 years among women born between 1915 and 1919 to 37.7 years among the 1935-1939 cohort (p = 0.0001). These findings have implications for women's lifetime exposure to circulating endogenous hormones.