Early reproductive characteristics, including menarcheal age, may have significant effects on later health outcomes. While early exposure to ovarian hormones may influence the risk for certain diseases, the degree to which this exposure is mediated by other factors is not well understood. Research on secular trends in age at menarche and subsequent outcomes in women's health across the lifespan can help to clarify the importance of age at menarche. Since 1934, the TREMIN Research Program on Women's Health has collected longitudinal data on menstruation and reproductive health. Recently we examined the age at menarche across the entire population of TREMIN participants, across the two major cohorts of this research program (participants initially enrolled in the 1930s and in the 1960s), and within families where mothers and daughters participated in the study. Our results indicated that age at menarche among the 4,444 mainly European American girls in our sample decreased in the first part of the twentieth century, and then began to increase in the late 1940s. The comparison of age at menarche across the two major cohorts reinforced this analysis: age at menarche was slightly but significantly lower (P < .001) in the second cohort (12.66), most of whom were born in the 1940s, than in the first cohort (12.89), those women born before 1920. In analyzing data from 380 mother-daughter pairs, there was no significant difference between mothers' and daughters' mean age at menarche and no significant correlation between them.