The current study confirmed with increased sample sizes our preliminary findings of event-related potential (ERP) abnormalities in obsessive-compulsive disorder (OCD) (Towey et al., 1990) and examined their relationship to symptom severity, treatment response, and neurological soft signs. Unmedicated patients (n = 17) showed larger negativities in N200 and slow wave regions than normal control subjects (n = 16) to correctly detected auditory "oddball" stimuli. N200 amplitude was larger over left than right hemispheres of OCD patients, but not normal control subjects. Greater N200 amplitude correlated with less severe obsessions, better response to subsequent treatment with serotonin reuptake blockers, and fewer neurological soft signs in OCD. With increased task difficulty, N200 and P300 latencies lengthened for normal subjects, but not for OCD patients. The abnormal ERP pattern in OCD supports hypotheses of cortical hyperarousal and overfocused attention.