Objective: This study examined associations between age of menarche and psychosocial outcomes in early adulthood, including sexual behavior, mental health, criminal behavior, and education/employment, to identify the possible causal role of earlier age of menarche in increasing risks of adverse outcomes. Method: Data were gathered from 497 female members of a New Zealand longitudinal birth cohort. Outcome measures (to age 18 years) were as follows: pregnancy; sexually transmitted infection (STI); "DSM-IV" major depression, anxiety disorder, alcohol dependence and conduct disorder/ASPD; self-reported crime; convictions; leaving school without qualifications; and unemployment. Associations between age of menarche and outcome measures were then adjusted for confounding factors spanning family background, family functioning, and individual factors. Additional analyses examined sexual behavior outcomes to age 21, and all other outcomes to age 30 years. Results: Analyses showed that sexual behavior outcomes (pregnancy, STI) were significantly associated with age of menarche, even after controlling for confounding factors. Anxiety disorder was marginally (p less than 0.10) associated with age of menarche. In addition, pregnancy during ages 18 to 21 years was significantly associated with age of menarche. There were no significant associations between age of menarche and outcomes at any other age (to age 30). Conclusions: The study showed that females reaching menarche at an earlier age were at increased risk of early pregnancy, and STI by age 18. The present study found little evidence to suggest that age of menarche was related to mental health, criminal behavior, or education/employment outcomes for this cohort. (Contains 4 tables.)