The authors used PET to show that the abnormal pattern of cerebral blood flow (CBF) activation described in PD is normalized by electrical stimulation of subthalamic nucleus. Improvement in bradykinesia correlated with increase in CBF due to stimulation in the supplementary motor area and anterior cingulate cortex. Compared to unilateral stimulation, bilateral stimulation induced a greater extent of activation in bilateral cortical areas and additional bilateral activation of globus pallidus.