Substance use disorders and other mental health problems are prevalent among juvenile justice populations. The presence of co-occurring substance use and mental health problems is associated with poorer treatment outcomes and higher recidivism rates. As such, it is imperative that adolescents with co-occurring conditions be correctly assessed. The current study examined the ability of the PADDI to identify co-occurring mental health and substance use problems and to provide an assessment of the interrelationships between the conditions. Participants were 284 adolescents recruited from several juvenile justice settings who completed the PADDI, which gathered information about substance use, mental health conditions, and related experiences. Results of the analysis indicated that the PADDI subscales had adequate to excellent internal consistency reliability. Analysis of the relationships between the severity levels of various conditions indicated that although some conditions correlated highly with respect to severity, conduct disorder and substance dependence did not correlate on measures of severity. However, the high prevalence rate for both conditions may explain the lack of apparent relationship in level of severity. These preliminary findings indicate that the PADDI is a reliable diagnostic instrument for the assessment of various mental health problems in juvenile justice populations. Future research should assess the test-retest reliability of the PADDI. Tables, references