Previous research has indicated that a large number of adolescents with mental health problems have had contact with the juvenile justice system. Identification and assessment of mental health problems, as well as substance abuse problems, are critically important to implementing effective interventions and treatments. The current study administered the Practical Adolescent Dual Diagnostic Interview (PADDI) to 252 adolescents who were consecutively admitted to 2 juvenile detention centers in Maine; the goal was to measure the prevalence of co-occurring mental health and substance use disorders among male and female juvenile offenders. Results of statistical analyses indicated that 90 percent of the juveniles were positive for some type of mental health or substance use disorder. Most adolescents showed signs of multiple mental health and substance use problems. Females had significantly greater mental health problems and maltreatment histories. The findings suggest that juvenile offender treatment programs should be prepared to provide both mental health and substance use treatments. Tables, references