This study uses a cohort-sequential longitudinal design to examine the patterns of change and codevelopment of anxiety, depression, and oppositional defiant symptoms (ODS) from late adolescence to young adulthood. Four waves of data were collected biennially by individual interview with a random, community-based sample of 662 youth ages 12 to 18 years at Time 1 (18-26 years at Time 4). Using latent growth curve modeling, we examined co-occurring changes in the levels, rates of change, and variability in symptoms of anxiety, depression, and oppositional defiance. Sex differences were also assessed. Levels of anxiety, depression, and ODS were correlated at each time point. Moreover, adolescents with high initial levels in one domain tended to have high initial levels in the other domains. In addition, increases in depressive symptoms were significantly correlated with increases in anxiety and in ODS, but adolescent levels of symptoms did not predict increases over time. Symptoms of anxiety (for female and male individuals) and depression (for male individuals) continue to increase in young adulthood, whereas ODS stabilize or decline. Adolescent levels of these problems have a significant impact on later levels, suggesting that preventive interventions may be needed in adolescence to defer negative consequences of mental health problems in young adults. (Contains 4 tables and 1 figure.)