Menarcheal age of a sample of Portuguese university students (n = 3,366), born between 1972 and 1983, age 18–23 years, was analyzed. The influence of parents' educational level and occupation, family size, birth order, and degree of urbanization of girl's locality of residence during childhood and adolescence were analyzed as well as secular trend in the Portuguese population. Mean age at menarche for girls born in 1983 was 12.32 years. Parents' educational level and occupation did not show any significant influence on mean age at menarche. Place of residence during childhood years and adolescence showed a significant effect on mean age at menarche, with girls from rural places with a later age at menarche than those who spent their childhood or adolescence in urban areas (P ≤ 0.01 and P ≤ 0.05 for childhood and adolescence, respectively). Family size and birth order showed the highest effect. Girls born in small families, with one child, matured earlier (12.32 years) than those born in large families with four or more children (12.67 years), (P ≤ 0.01). Also, girls that were first-born had an earlier menarche (12.41 years) than those who were third or later (12.58 years, P ≤ 0.01). Regression analysis selected family size and place of residence in childhood as the most important determinants of mean age at menarche in our university students. In this sample, from 1972 to 1983, mean age at menarche remained stable. When we considered published data from all the Portuguese population we found a decrease in mean age at menarche from 15 years for girls born in 1880 to 12.44 for those born in the 1980s. This decrease was the result of great improvements in the social and economic living conditions that occurred in Portugal especially after the 1970s concerning nutrition and health care, among many other environmental factors. Am. J. Hum. Biol. 15:415–427, 2003. ©2003 Wiley-Liss, Inc.