CONTEXT Psychiatric disorders are prevalent among incarcerated juveniles. Most juveniles eventually return to their communities, where they become the responsibility of the community mental health system. However, no large-scale study has examined psychiatric disorders after youth leave detention. OBJECTIVE To examine changes in the prevalence and persistence of psychiatric disorders during the 5 years after detention, focusing on sex and racial/ethnic differences. DESIGN Prospective longitudinal study with up to 5 interviews (1829 youth: 1172 males and 657 females). To ensure representation of key demographic subgroups, the randomly selected sample was stratified by sex, race/ethnicity (African American, non-Hispanic white, and Hispanic), age, and legal status (juvenile or adult court). SETTING The Northwestern Juvenile Project, sampling youth from the Cook County Juvenile Temporary Detention Center, Chicago, Illinois. PARTICIPANTS Detained youth, aged 10 to 18 years at baseline interview. MAIN OUTCOME MEASURES At baseline, the Diagnostic Interview Schedule for Children Version 2.3. At follow-up interviews, the Diagnostic Interview Schedule for Children Version IV (Child and Young Adult versions) and the Diagnostic Interview Schedule Version IV (substance use disorders and antisocial personality disorder). RESULTS Five years after baseline, more than 45% of males and nearly 30% of females had 1 or more psychiatric disorders with associated impairment. More than 50% of males and more than 40% of females had 1 or more psychiatric disorders without impairment. Substance use disorders were the most common; males, however, had higher rates over time (5 years after baseline, adjusted odds ratio [AOR], 2.61; 95% CI, 1.96-3.47). Non-Hispanic whites and Hispanics also had higher rates of substance use disorders vs African Americans (AOR, 1.96; 95% CI, 1.54-2.49 and AOR, 1.59; 95% CI, 1.24-2.03). Females had higher rates of major depression over time (AOR, 1.59; 95% CI, 1.22-2.08). CONCLUSIONS Although prevalence rates of most psychiatric disorders declined as youth aged, a substantial proportion of delinquent youth continue to have disorders. There are notable sex and racial/ethnic differences in the prevalence and persistence of psychiatric disorders in this population.