Prospective relations between personality traits and mental disorders were assessed in a longitudinal study of a representative birth cohort of young men and women from Dunedin, New Zealand. Personality traits were assessed via self-report questionnaire at age 18, and mental disorders were assessed via diagnostic interview at both ages 18 and 21. High “negative emotionality” (a propensity to experience aversive affective states) at age 18 was linked with affective, anxiety, substance dependence, and antisocial personality disorders at age 21 when corresponding mental disorders at age 18 were controlled. Low “constraint” (difficulty inhibiting the expression of affect and impulse) at age 18 was linked with substance dependence and antisocial personality disorders at age 21 when corresponding mental disorders at age 18 were controlled. The findings are discussed in terms of their implications for the development and treatment of mental disorders in young adulthood.