Objective Unlike adolescents with adolescent‐onset (AO) disruptive behavior, adolescents with early‐onset (EO) disruptive behavior may not benefit from treatment. Method Using Symptom Checklist (SCL‐90‐R) ratings at admission and discharge of adolescent inpatients with EO (n = 85) and AO (n = 60) disruptive behavior treatment outcome was determined by (a) a change in mean scores and (b) the Reliable Change Index. For a subgroup, ratings on the Satisfaction Questionnaire Residential Youth Care for Parents (n = 83) were used to verify the treatment outcome. Results Inpatients with EO disruptive behavior had a higher risk of dropout (44.4%) from treatment than the AO group (24.7%). Among the treatment completers, both onset groups reported improvements on the SCL‐90‐R, with 26.9% recovering and 31.7% improving. Inpatients who reported improvement were mostly rated as improved by their parents (r = .33). Conclusion As EO inpatients are more likely to drop out, interventions should aim at motivating youngsters to continue treatment, particularly given the poor outcome in this group. Treatment may benefit both groups because those EO youths who stayed in treatment improved to the same extent as AO inpatients.