OBJECTIVE: To compare detection rates, service use, and outcomes of substance use disorder (SUD) in adolescents seeking mental health treatment. METHOD: Adolescents (n = 237) and their parents or caregivers completed parallel, self-administered versions of the Adolescent Treatment Outcomes Module (ATOM) at intake and 6-month follow-up. SUD was assessed using the Diagnostic Interview Schedule for Children (DISC). RESULTS: Although 42 (16.6%) adolescents seeking mental health treatment met DISC criteria for any SUD at baseline, clinicians detected only 19 of these (kappa = 0.51). Sensitivity and specificity were high for a screening tool for adolescent SUD (95.2 and 82.0, respectively). Adolescents with undetected SUD were less likely to receive SUD services and to have more legal problems at 6-month follow-up compared to adolescents with detected SUD. CONCLUSIONS: SUD rates may be high in adolescents seeking mental health treatment, confirming the need for routine screening in this population. Results confirm that a large gap exists between the need for and access to SUD treatments, which may contribute to poorer outcomes for all adolescents with SUD.