Neuroimaging studies in recent years suggest that cortical hyperactivity associated with more aroused cognitive processes and overfocussed attention is involved in the pathogenesis of obsessive-compulsive disorder (OCD), which was electrophysiologically supported by an own pilot-study in a small sample of stabilized OCD patients. To replicate this first finding, the hypothesis of cortical hyperactivity was studied by measuring auditory event-related P300 subcomponents, especially the amplitude of the P3a and P3b subcomponent, in a large sample of acutely ill and unmedicated patients with OCD. The P300 of 63 patients with OCD (30 males, 33 females, 33.7+/-10.2 years old; 25.4+/-5.4 points at Yale-Brown-Obsessive-Compulsive-Scale (Y-BOCS)) was separated with dipole source analysis (BESA) into their subcomponents P3a and P3b, and compared to the P300 subcomponents of 63 gender and age matched healthy controls. No difference in the amplitude of P3a was found, but OCD patients had significantly larger amplitudes of P3b than the healthy controls, which replicates the results of the pilot study. Once again, our findings point to a hyperactivated cortical state also of temporo-parietal and hippocampal regions in OCD.