Several lines of evidence suggest that frontal lobe dysfunction may underlie obsessive-compulsive disorder (OCD). Eighteen patients with OCD were compared with 18 normals matched for age, gender, handedness, and education on a Go-NoGo task. Visual evoked potentials were measured during the task. Topographic evoked potential mapping revealed significantly smaller P300 magnitudes in orbital frontal areas in the OCD patients. Results are compared with those from studies using other methodologies, and etiological implications are discussed.