OBJECTIVE: The authors’ goal was to determine the prevalence of major mental disorders and substance abuse in adolescents admitted to a juvenile detention center. METHOD: As part of a routine mental health screening, modules from the Diagnostic Interview Schedule for Children were administered to 50 youths (11–17 years old) at an urban juvenile detention center. RESULTS: A high rate of affective disorder (42%) was found among these adolescents: 10 (20%) met criteria for mania, another 10 met criteria for major depressive disorder, and one met criteria for bipolar disorder, mixed type. Thirty (60%) met criteria for conduct disorder, and very high rates of alcohol, marijuana, and other substance dependence were found. There was a strong association between affective disorder and conduct disorder; adolescents with mania had much higher rates of reported abuse of substances other than alcohol or marijuana. CONCLUSIONS: Juvenile offenders have high rates of affective disorder. Further studies are needed to examine the relationship of affective disorder to substance abuse as well as to antisocial behavior.