This paper examines the effects of living in a stepfamily during childhood and adolescence on a range of psychosocial outcomes at age 18 years. Data collected during an 18-year longitudinal study were used to examine a sample of 907 children with respect to: exposure to living in a stepfamily during the period from age 6 to 16 years; measures of psychosocial outcomes including mental health, antisocial behaviour, substance use, restricted life opportunities, and sexual risk-taking at age 18 years; and measures of prospectively collected confounding factors. The analyses revealed that children exposed to living in a stepfamily for the first time between ages 6–16 years had elevated risks of a range of psychosocial outcomes at 18 years. These included elevated risks of: (1) juvenile offending; (2) nicotine dependence; (3) abuse or dependence on illicit substances; (4) leaving school without qualifications; (5) early onset of sexual activity; and (6) multiple sexual partners. However, these risks were reduced substantially when psychosocial outcomes were adjusted for the confounding effects of antecedent factors such as: family socioeconomic characteristics; family history of instability, adversity, and conflict; mother's age, religiosity, and smoking; child gender; and preexisting child conduct and attentional problems. After adjustment, the odds ratios between exposure to a stepfamily and adolescent outcomes were nonsignificant. Additional analysis revealed that there were no significant differences in outcomes for boys and girls exposed to stepfamilies. It was concluded that although young people exposed to living in a stepfamily had increased risks of poor psychosocial outcomes, much of this association appeared to be spurious, and arose from confounding social, contextual, and individual factors that were present prior to the formation of the stepfamily.