BACKGROUND: Study results of event-related potential in obsessive compulsive disorder (OCD) remain controversial, potentially as a result of different instruments utilized and their differing technical characteristics.OBJECTIVE: To investigate the differences in several common event-related potentials, i.e. contingent negative variations, P300, and mismatch negativity (MMN), in OCD patients, depression patients, generalized anxiety disorder (GAD) patients, and healthy controls.DESIGN, TIME AND SETTING: A case-control study was performed in the Department of Electrophysiology, Shanghai Mental Health Center from May 2002 to December 2005.PARTICIPANTS: A total of 38 OCD patients, 20 depression patients, and 18 GAD patients, who were diagnosed according to the criteria of Chinese Classification of Mental Disorders (Version 3), formulated by the Chinese Psychiatry Association, were selected from the Outpatient Department of Shanghai Mental Health Center. Patients with two or more the above diseases were excluded. In addition, 28 healthy people, gender and age matched, were selected as controls.METHODS: Contingent negative variations, P300, and MMN were recorded by a Nicolet Spirit Instrument. All electrodes were attached at Cz according to the International 10-20 system, with the mastoid leads as reference and Fpz as ground. MAIN OUTCOME MEASURES: Amplitude and latency of contingent negative variations, P300, and MMN.RESULTS: The contingent negative variations, P300, and MMN were different (P < 0.01). OCD patients showed an increased M1 amplitude compared with controls, depression, and GAD patients (P < 0.01). Target P300 amplitudes were significantly lower in OCD, depression, and GAD patients compared with controls (P < 0.01). Moreover, N2 latency and latency of MMN were prolonged in OCD and depression groups compared with controls (P < 0.05).CONCLUSION: Event-related potentials were different in depression, GAD, and OCD patients and healthy controls. In particular, OCD patients exhibited unique characteristics.