The mean age at menarche in American girls enrolled in high schools and a college in northeastern Arkansas was found to be 12.63±0.03 years. The onset of menstruation was most frequent in the months of May and June. The findings in this study indicate a significantly earlier onset of menstruation when compared to the results of earlier studies in this country. The menarchial age of girls residing in other US communities as well as in other countries are reviewed and reasons for the world-wide trend to earlier menarche discussed. The results of this study suggest that environmental influences, particularly socioeconomic factors, are among the most important of the several factors which influence menarche.