Kazuhiko Yamamuro,1 Koji Okada,2 Naoko Kishimoto,1 Toyosaku Ota,1 Junzo Iida,3 Toshifumi Kishimoto1 1Department of Psychiatry, Nara Medical University School of Medicine, 2Department of Psychiatry, Jingumaecocorono-Clinic, 3Faculty of Nursing, Nara Medical University School of Medicine, Kashihara, Japan Aim: Earlier brain imaging research studies have suggested that brain abnormalities in obsessive-compulsive disorder (OCD) normalize as clinical symptoms improve. However, although many studies have investigated event-related potentials (ERPs) in patients with OCD compared with healthy control subjects, it is currently unknown whether ERP changes reflect pharmacological and psychotherapeutic effects. As such, the current study examined the neurocognitive components of OCD to elucidate the pathophysiological abnormalities involved in the disorder, including the frontal-subcortical circuits.Methods: The Yale-Brown Obsessive-Compulsive Scale was used to evaluate 14 adult patients with OCD. The present study also included ten age-, sex-, and IQ-matched controls. The P300 and mismatch negativity (MMN) components during an auditory oddball task at baseline for both groups and after 1 year of treatment for patients with OCD were measured.Results: Compared with controls, P300 amplitude was attenuated in the OCD group at Cz and C4 at baseline. Pharmacotherapy and psychotherapy treatment for 1 year reduced OCD symptomology. P300 amplitude after 1 year of treatment was significantly increased, indicating normalization compared with baseline at Fz, Cz, C3, and C4. We found no differences in P300 latency, MMN amplitude, or MMN latency between baseline and after one year of treatment.Conclusion: ERPs may be a useful tool for evaluating pharmacological and cognitive behavioral therapy in adult patients with OCD. Keywords: obsessive-compulsive disorder, event-related potentials, P300, mismatch negativity, improvement