ABSTRACT: The objectives of the study were to identify factors associated with adolescent emergency department (ED) visits for substance abuse, including those complicated by mental health (dual diagnosis), and to analyze their effect on ED length of stay (LOS) and disposition.We performed a secondary analysis of ED visits by adolescents (age, 11-24) using the National Hospital Ambulatory Medical Care Survey (1997-2010) to identify visits for mental health, substance use, and dual diagnosis. Univariate and multivariate statistics were used to analyze demographic and visit-level factors, factors associated with substance use and dual diagnosis visits, as well as the effects of substance use and mental health conditions on ED LOS and disposition.Substance use and mental health accounted for 2.1% and 4.3% of all adolescent visits, respectively, with 20.9% (95% confidence interval [CI], 18.3%-23.5%) of substance abuse visits complicated by mental health. The factors significantly associated with substance use include the following: male sex, urban location, West region, ambulance arrival, night and weekend shift, anxiety disorders, mood disorders, and psychotic disorders. Additional LOS was 89.77 minutes for mental health, 71.33 minutes for substance use, and 139.97 minutes for dual diagnosis visits, as compared with visits where these conditions were not present. Both mental health and substance use were associated with admission/transfer as compared with other dispositions as follows: mental health odds ratio (OR), 5.93 (95% CI, 5.14-6.84); illicit drug use OR, 3.56 (95% CI 2.72-4.64); and dual diagnosis OR, 6.86 (95% CI, 4.67-10.09).Substance abuse and dual diagnosis are common among adolescent ED visits and are strongly associated with increased use of prehospital resources, ED LOS, and need for hospitalization.