The spatiotemporal pattern of event-related desynchronization (ERD) during the motor preparation period preceding a self-paced voluntary wrist-flexion was compared in two groups of 10 right and 10 left hemiparkinsonian patients, before and after chronic administration of L-DOPA. ERD was computed in the 9-11 Hz frequency band from 11 source derivations covering the medial frontocentral, central and parietocentral areas, during two successive left and right experimental conditions (100 self-paced wrist flexions). In the two groups ERD appeared with a shorter latency over the contralateral primary sensorimotor area, when the movements were performed with the akinetic hand. After L-DOPA administration, earlier ERD onset before the movement was observed in both groups over the contralateral and ipsilateral central and parietocentral areas. A medial frontocentral ERD distribution was also observed before the onset of movement, especially in the right hemiparkinsonian group. Delayed ERD onset, which shows that programming of movement is affected in Parkinson's disease, may be partially corrected by L-DOPA therapy.