This study compared the pretreatment characteristics and posttreatment outcomes of substance-abusing adolescents with and without comorbid mental disorders in the Drug Abuse Treatment Outcome Studies for Adolescents. Subjects (N = 992) were sampled from 23 adolescent drug treatment programs across three modalities (residential, short-term inpatient, outpatient drug-free). Nearly two thirds (64%) of the sample had at least one comorbid mental disorder, most often conduct disorder. Comorbid youth were more likely to be drug or alcohol dependent and had more problems with family, school, and criminal involvement. Although comorbid youth reduced their drug use and other problem behaviors after treatment, they were more likely to use marijuana and hallucinogens, and to engage in illegal acts in the 12 months after treatment, as compared with the noncomorbid adolescents. Integrated treatment protocols need to be implemented within drug treatment programs in order to improve the outcomes of adolescents with comorbid substance use and mental disorders.