This study aimed at investigating whether one of the key deficits in Attention Deficit Hyperactivity Disorder (ADHD), slow response inhibition, predicted the response to methylphenidate (MPH) treatment. In order to address this issue, we used Stop Signal Reaction Times (SSRTs) measured at baseline in 20 medication‐naïve boys with ADHD as predictor, and parent and teacher ratings that were collected during a double‐blind, placebo‐controlled titration trial of MPH in the same group as outcome measures. Parent and teacher ratings were collected on primary scales, measuring ADHD symptoms, and secondary scales, measuring oppositional and disruptive behaviour. Placebo response and ADHD/Oppositional Defiant Disorder symptom severity at baseline were controlled for in the analyses. The SSRT did not predict the MPH response as measured by parent ratings, but it did predict the MPH response as measured by teacher ratings. This effect was specific for the ADHD scales. The slower SSRTs were, the less children benefited from MPH. Moreover, children with longer SSRTs needed higher doses of MPH for optimal symptom relief than children with shorter SSRTs. These findings have implications for clinicians who face the decision of which MPH dose to prescribe.