The purpose of the current study was to evaluate whether error monitoring difficulties persist in adults with attention deficit hyperactivity disorder (ADHD) using the event-related potential (ERP) methodology. Adults with ADHD and age-matched healthy controls executed a visual Go/No-Go task with 25% No-Go trials. Performance and ERP correlates of error monitoring were compared between groups. At the performance level no difference was noted between groups. However, exploring the error-related potentials revealed that the error-related negativity (ERN) was the same for both groups, but that adults with ADHD showed a smaller error positivity (Pe). Based on these findings, we conclude that adults with ADHD are normal in early automatic error detection, but are deviant in later conscious evaluation of the error. The findings add to the increasing evidence supporting disturbances in error monitoring in ADHD and show that these problems may persist in adulthood ADHD.