To estimate the current age at menarche in 2 urban areas from Mexico and to analyze secular trends in the age at menarche in Mexico City.Cross-sectional survey and secondary data analyses.(1) Samples of female students from Mexico City and from the city of Xalapa; and (2) national health surveys.(1) Pre- and postmenarcheal students, aged 8-17 years; and (2) data for women born between 1904 and 1999 who grew up in Mexico City were obtained from 3 relevant national health surveys.(1) Students were asked if they had had their first period and when it occurred; (2) information on year of birth and age at menarche for the remaining women was obtained from the national surveys; data were divided in groups according to the decade in which the women were born.(1) The estimated mean age at menarche was 11.40 and 11.34 years in Mexico City and Xalapa respectively. (2) There were significant differences in age at menarche among women who were born in the different decades analyzed. Women who were born in the 1990s or later reported an earlier age at menarche than those who were born from the 1940s to 1980s (P < .0001).These results could be helpful to health and education professionals who should take into account that because there is currently a widening gap between the physical and psychosocial maturity of girls, they might need special support.