Children with ADHD make more errors than control children in response-conflict tasks. To explore whether this is mediated by enhanced sensitivity to conflict or reduced error-processing, task-related brain activity (N2, Ne/ERN, Pe) was compared between 8- to 12-year-old children with ADHD and healthy controls during performance of a flanker task. Furthermore, effects of methylphenidate were investigated in ADHD children in a second study. ADHD children made more errors, especially in high-response-conflict conditions, without showing post-error slowing. N2 amplitudes were enhanced on trials resulting in an error response, Ne/ERN amplitude was unaffected and Pe amplitude was reduced in the ADHD group. Methylphenidate reduced errors in both low- and high-conflict conditions and normalized Pe amplitudes in children with ADHD. It was concluded that the inaccurate behaviour of ADHD children in conflict tasks might be related to reduced error-awareness and higher sensitivity to response conflict. Methylphenidate's ameliorating effects might be established through its influence on brain networks including posterior (parietal) cortex, enabling children with ADHD to allocate more attention to significant events.