- The improved nutrition and socioeconomic status of the population in industrialized countries has resulted in a decrease in the mean age at menarche. This trend raises the question of whether cigarette smoking and the use of oral contraceptives, health behaviors often adopted during adolescence, may also be starting at a younger age. Cigarette smoking and use of oral contraceptives are a public health concern because they pose an increased risk for development of chronic diseases, particularly in combination. This study was designed to identify secular trends in age at menarche, at first cigarette, and at first use of oral contraceptives among a large population-based sample of young Israeli women and to assess whether these trends are associated with sociodemographic factors. A systematic, population-based survey used data obtained from female recruits to the Israel Defense Force from 1986 to 2000. During the study period, 11,392 questionnaires were collected from Jewish women aged 18 to 19 years. Participants were interviewed concerning geographic origin and level of education, father's geographic origin and level of education, current smoking status, use of oral contraceptives, and recalled age at first menstruation, first cigarette, and first use of oral contraceptives. Reported mean age (-/+ D) at menarche showed a monotonic trend of decreasing over time, from 13.41 (-/+ .30) years for women born before 1970 to 13.03 (-/+ 1.28) years for those born after 1978 (P < .001). Women born after 1978 were twice as likely to experience menarche by the age of 11 as those born prior to 1970 (odds ratio 2.0; 95% confidence interval, 1.41-2.82). Significant trends toward younger age at first use were observed for cigarettes and oral contraceptives. The trends of earlier age at menarche, first cigarette, and first use of oral contraceptives suggest health behaviors among young women that may herald increased chronic disease morbidity in the future. These trends indicate the need for further investigation and preventive measures aimed at this population.