Positron emission tomography allows a quantitative assessment of the impact of functional neurosurgery in Parkinson's disease (PD) by measuring regional cerebral flow and glucose and oxygen consumption as indicators of metabolic activity of specific brain regions. PET can also be used to study the dopaminergic nigrostriatal system, and therefore serves as a surrogate marker of the evolution of striatal grafts for PD. Pallidotomy has been associated with increased activation of premotor areas (supplementary motor area and dorsolateral prefrontal cortex) and reduced hyperactivity of the lentiform nucleus (augmented preoperatively). Pallidal (GPi) and subthalamic (STN) stimulation also increase activation of premotor areas but decrease activation of primary motor area. Suppression of unilateral tremor with thalamic stimulation is associated with a reduction in cerebellar blood flow. These main findings are in keeping with the general notion that increased activity in the STN GPi projection is directly implicated in the pathophysiology of PD. Surgical blockage of these output nuclei leads to partial restoration of cortical physiology.