The event-related desynchronization (ERD) to voluntary movemtnt is an indicator of cortical activation with a high time resolution and a specific spatial representation. We have evaluated 10 patients affected by Parkinson's disease (PD), free from L-dopa treatment for at least 12 hours, and 10 control subjects. Each subject underwent ERD examination during self-paced movement (SPM) and during contingent negative variation (CNV) paradigms. ERD was measured as the percentage decrease of alpha band power and calculated for frequency bands of 1 Hz, ranging between 8 and 12 Hz. For group comparisons, the frequency showing the highest ERD was selected for each subject and for each side. In the control group. ERD in the CNV paradigm began over the contralateral centroparietal electrodes 1475 ms before movement onset of the right hand and 1375 ms for the left. In the SPM paradigm, ERD started over the contralateral central electrodes 2150 ms and 1775 ms before movement onset of the right and left hand, respectively. In the PD group, ERD started over the contralateral central areas 800 ms and 475 ms before movement onset of the right and left hand, respectively, for CNV paradigm and 1200 ms and 750 ms for the right and left hand, respectively, for SPM paradigm. Therefore, contralateral ERD began closer to movement onset in PD compared with the control group in both paradigms. ERD over the sensorimotor areas ipsilateral to the movement was not significantly different in PD compared with the control group. The finding of delayed contralateral ERD in PD is according to the view that functional cortical activation related to movement preparation is impaired in PD. The lack of group differences in the onset of ipsilateral ERD, which appears close to movement execution than contralateral ERD both in normal subjects and in PD, suggests that different mechanisms may be involved in generating ERD over the hemispheres ipsilateral and contralateral to the movement, and that only the latter are impaired in PD.