BACKGROUND : Previous research has documented differences in health behaviours between men and women, with differential risks and health outcomes between the sexes. Although some sex-specific differences in health outcomes are caused by biological factors, many others are socially driven through gender norms. We therefore aimed to assess whether gender expression as an adolescent, determined by the degree to which an individual's behvaiours were typical of their gender, were associated with health behaviours and outcomes in adulthood. METHODS : In this prospective cohort study, we used data from the National Longitudinal Study of Adolescent to Adult Health, a nationally representative sample of US adolescents from whom data were collected during adolescence (ages 11-18 years) and adulthood (ages 24-32 years). We created a measure of gender expression that was based on the degree to which male and female adolescents and adults behave in stereotypically masculine (for men) or feminine (for women) ways relative to their same-gender peers. Adolescents were assessed for baseline sociodemographic characteristics and gender expression, and these participants were later assessed, during adulthood, for their gender expression and health behaviours and outcomes, which included depression, self-rated health, drug and alcohol use, cardiovascular risk factors, experience of sexual violence, diet, and obesity. These data were collected via surveys, except for body-mass index, cholesterol, and blood pressure, which were collected as biomarkers. RESULTS : Between April and December, 1995, self-reported data were collected from 10 480 female and 10 263 male adolescents; similar data were subsequently collected in several waves in this cohort, with a final collection between January, 2008, and February, 2009, when participants were aged 24-32 years. We used data from this final wave and from baseline, and our study represents a secondary analysis of these data. Of these participants, complete follow-up data from 6721 (80%) adult women and 5885 (80%) adult men were available. Gender expression was stable for men and women from adolescence to adulthood. High masculinity (vs low masculinity) in adolescent and adult men was positively associated with smoking in the past month, use of marijuana and recreational drugs, prescription drug misuse (adult gender expression only), and consumption of fast food and soda (adolescent gender expression only) in the past week. However, higher masculine gender expression in adult men was negatively associated with diagnosed depression and high cholesterol in adulthood, and masculine gender expression in adolescent and adult men was negatively associated with high blood pressure in adults. High femininity (vs low femininity) in adolescent or adult women was positively associated with high cholesterol and blood pressure (both adult gender expression only), depression, migraines (adult gender expression only), and physical limitations (ie, health problems that limited their daily activities). However, higher femininity in adolescence was negatively associated with self-rated good health in adulthood. Although feminine gender expression in adolescents was predictive of adult recreational and prescription drug and marijuana use and experience of sexual violence, feminine gender expression in adulthood was negatively associated with adult substance use and experience of sexual violence, suggesting that expressions of femininity typical of adolescents impart risks that expression of femininity as an adult does not. Individuals who are highly masculine or feminine seem to be at greatest risk of adverse health outcomes and behaviours. CONCLUSIONS : We found compelling evidence that adolescent gender expression is correlated with health in adulthood independently of gender expression as an adult. Although more research is needed to identify causal mechanisms, our results suggest that those designing health behaviour interventions should carefully consider integrating gender transformative components into interventions. BACKGROUND : Eunice Kennedy Shriver National Institute of Child Health and Human Development, Gender Equality, Integrated Delivery, HIV, Nutrition, Family Planning, and Water Sanitation and Hygiene Program Strategy Teams (Bill and Melinda Gates Foundation).