Positron emission tomography was used to measure global and local blood flow in 11 patients with hemiparkinsonism (before rind after an acute oral dose of L-dopa) and in 26 normal subjects. Global hemispheric blood flow was not significantly different between the patients [45 ± 11 ml/(min.100 g)] and the controls [49 ± 8 ml/(min.100 g)]. After L-dopa, the patients’ mean global hemispheric flow did not change. Measurements of local blood flow from specific, anatomically defined cortical and basal ganglia regions were performed using a newly developed stereotactic localization technique. Before L-dopa, mesocortical blood flow contralateral to the patients’ symptoms was significantly less than controls (p < 0.003), suggesting a specific abnormality in the cortical dopaminergic projection from the ventral tegmental area. In addition, right and left pallidal blood flow were significantly less tightly coupled in patients than controls (p = 0.0312). After L-dopa, the mesocortical blood flow remained below normal, whereas pallidal blood flow was no longer significantly different from controls (p > 0.05).