Objective: To study whether behavioral and emotional problems during childhood predicted early sexual debut, risky sex at age 21 years, and sexually transmitted infections up to age 21 years. Some possible mediational pathways were also explored. Method: Participants were enrolled in the Dunedin Multidisciplinary Health and Development Study (n = 1,037), a prospective, longitudinal study of a New Zealand birth cohort born in 1972-1973. Data obtained at ages 5, 7, 9, 11, 13, 15, and 21 years were used. Adjustment was made for gender, socioeconomic status, parenting factors, and residence changes. Results: High levels of antisocial behavior between age 5 and 11 years were associated with increased odds of early sexual debut (adjusted odds ratio [AOR] 2.17, 95% confidence [CI] 1.34-3.54) and risky sex (AOR 1.88, 95% CI 1.04-3.40). No relationship was observed between hyperactivity and later sexual health outcomes. In contrast, high levels of anxiety were associated with reduced odds of risky sex (AOR 0.45, 95%CI 0.25-0.80) and sexually transmitted infections (AOR 0.34, 95% CI0.17-0.70). Involvement with delinquent peers explained some of the association between antisocial behavior and early sexual debut and risky sex. A poor relationship with parents also explained some of the association between antisocial behavior and early sexual debut. Conclusions: The findings demonstrate links between behavioral and emotional problems occurring early in life and later deleterious sexual health outcomes. Targeting antisocial behavior and teaching accurate appraisals of danger during childhood may help mitigate these negative consequences.