Objective: Overactive performance monitoring, as measured by the error-related negativity in the event-related brain potential, represents one of the most robust psychophysiological alterations in obsessive-compulsive disorder (OCD). It has been proposed as an endophenotype for OCD because it is heritable and more prevalent in families of OCD patients. Consistent with this notion, it is also independent of symptom profile and symptom severity in cross-sectional studies. Longitudinally, it has been shown to be state independent in pediatric patients with OCD. The purpose of the present study was to investigate the state dependency of error monitoring by examining adult OCD patients before and after symptom reduction through cognitive-behavioral therapy (CBT).Method: Error-related and correct-related negativity as electrophysiological indicators of performance monitoring were recorded from 45 OCD patients and 39 healthy comparison subjects while performing a flanker task. Patients were assessed before starting and after completing a standard 30-session CBT, including exposure and response prevention, and healthy comparison subjects were tested after a comparable time interval.Results: Pretreatment, patients with OCD were characterized by enhanced error-related and correct-related negativity compared with healthy comparison subjects. This difference persisted after treatment when symptoms were substantially reduced. There was no significant correlation between symptom improvement and changes in performance monitoring and no difference in performance monitoring between treatment responders and nonresponders.Conclusions: This is the first longitudinal study in adult OCD patients showing stability of enhanced error monitoring following successful symptom reduction through CBT. It supports the hypothesis that overactive performance monitoring is an endophenotype that indicates vulnerability for OCD.