Evidence suggests that a hyperactive frontal-striatal-thalamic-frontal circuit is associated with the symptoms of obsessive-compulsive disorder (OCD), but there is little agreement about the function of the exaggerated activity. We report electrophysiological evidence suggesting that part of this system monitors events and generates error signals when the events conflict with an individual's internal standards or goals. Nine individuals with OCD and 9 age-, sex-, and education-matched control participants performed a speeded reaction time task. The error-related negativity, an event-related brain potential component that reflects action-monitoring processes, was enhanced in the individuals with OCD. The magnitude of this enhancement correlated with symptom severity. Dipole modeling suggested that the locus of the enhancement corresponded to medial frontal regions, possibly the anterior cingulate cortex.