Objective: Inhibition deficits, including deficits in prepotent response inhibition and interference control, are core deficits in ADHD. The predictive value of prepotent response inhibition and interference control was assessed for outcome in a 10-week treatment trial with methylphenidate. Methods: Thirty-four children with ADHD (ages 8-12) received 10 weeks of methylphenidate treatment. At pretest prepotent response inhibition was assessed using the Stop-Signal Task; interference control was assessed using the Stroop Color-Word task. Methylphenidate was individually titrated to an optimal dose. Treatment outcome was assessed by parent- and teacher-rated ADHD behavior. Results: Only stop-signal reaction time of the Stop-Signal Task was a significant predictor of parent-rated levels of inattention and hyperactivity/impulsivity at outcome. Children with lower levels of inhibition showed worse outcome after 10 weeks of treatment, independent of medication dose. Conclusions: Low levels of prepotent response inhibition are associated with worse response to treatment with methylphenidate. Prepotent response inhibition may be an intermediate phenotypical predictor of treatment outcome. © 2012 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business.