While there has been a lot of focus on possible mental health and substance abuse problems among juvenile offenders, scant research has focused on its prevalence or on the co-occurrence of offending behavior, mental health problems, and substance abuse. As a result, efficacious mental health and substance abuse treatment for adjudicated youth has been lacking. In an effort to fill this void in the literature, the authors examined the mental health and substance abuse backgrounds of 155 youth who were admitted to 2 detention centers in a western State. The authors hypothesized that since these youths were selected because they required treatment-level intervention in at least one problem area, elevated levels of mental health symptoms and substance abuse among this sample was likely. Results of statistical analyses revealed that the hypothesis was correct; over one-third of the sample displayed mental health symptoms that exceeded the clinical range on at least one of the nine BSI subscales. Moreover, self-reported levels of substance abuse among the participants were significantly higher than that reported by the average youth population. Three patterns of co-occurrence were uncovered through the use of cluster analysis. Adolescents in two of the clusters displayed moderate mental health symptoms but differed in the level of substance abuse and delinquent behavior. The third group displayed serious mental health symptoms, which co-occurred with high rates of substance use and delinquent behavior. These results point to an emerging issue in treating offending youth, namely that the juvenile justice system needs to be prepared to treat multiple problems of mental health and substance abuse issues in juvenile offenders. Tables, references