Objectives: ADHD and reading disability (RD) are distinct disorders that often appear together. Individuals with both disorders are currently diagnosed based on questionnaires/behavioral performance. The present study aimed to determine whether ADHD alone differs from ADHD with RD in error monitoring, which is part of the executive system, as measured while reading. Method: Event-related potentials were recorded during a lexical decision task performed by children with comorbid ADHD and RD and children with ADHD. Results: Lower executive function and reading abilities were accompanied by decreased event-related potential components in participants with ADHD and RD, compared with participants with ADHD. Conclusion: Results suggest that the error monitoring activation can be used as a possible biomarker to objectively differentiate ADHD with RD from ADHD alone. (J. of Att. Dis. 2013; XX(X) 1-XX)