OBJECTIVE: Converging evidence in patients with obsessive-compulsive disorder (OCD) shows abnormalities of prefrontal areas and basal ganglia, which are also involved in motor control. Event-related desynchronization of mu and beta EEG rhythms is considered a correlate of motor activation during motor preparation and execution, followed by cortical idling or inhibition indicated by event-related synchronization. The authors investigated the circuits involved in motor behavior in OCD by using event-related desynchronization/synchronization. METHOD: Data on alpha and beta event-related desynchronization/synchronization with self-paced movement of the right thumb were obtained by using 29-channel EEG in 10 untreated OCD patients and 10 normal subjects. RESULTS: OCD patients showed delayed onset of mu event-related desynchronization with movement preparation and less postmovement beta synchronization, compared to normal subjects. CONCLUSIONS: Delayed event-related desynchronization in OCD is consistent with involvement of structures related to motor programming, such as basal ganglia. Lower levels of postmovement beta synchronization suggest impairment of the inhibitory system in OCD.