Objective: In this randomized controlled trial, we investigated the effectiveness of a school-based targeted intervention program for disruptive behavior. A child-focused cognitive behavioral therapy (CBT) program was introduced at schools in disadvantaged settings and with active teacher support (ATS) versus educational teacher support (ETS) (CBT + ATS vs. CBT + ETS). Method: Screening (n = 1,929) and assessment (n = 224) led to the inclusion of 173 children ages 8-12 years from 17 elementary schools. Most of the children were boys (n = 136, 79%) of low or low-to-middle class socioeconomic status (87%); the sample was ethnically diverse (63% of non-Western origin). Children received CBT + ATS (n = 29) or CBT + ETS (n = 41) or were entered into a waitlist control condition (n = 103) to be treated afterward (CBT + ATS, n = 39, and CBT + ETS, n = 64). Effect sizes (ES), clinical significance (reliable change), and the results of multilevel modeling are reported. Results: Ninety-seven percent of children completed treatment. Teachers and parents reported positive posttreatment effects (mean ES =.31) for CBT compared with the waitlist control condition on disruptive behavior. Multilevel modeling showed similar results. Clinical significance was modest. Changes had remained stable or had increased at 3-months follow-up (mean ES =.39). No consistent effect of teacher condition was found at posttreatment; however, at follow-up, children who received ETS fared significantly better. Conclusions: This study shows that a school-based CBT program is beneficial for difficult-to-reach children with disruptive behavior: The completion rate was remarkably high, ESs (mean ES =.31) matched those of previous studies with targeted intervention, and effects were maintained or had increased at follow-up. © 2013 American Psychological Association.