Thirty-six patients affected by Parkinson's disease were studied using single photon emission computed tomography (SPECT) and [99mTc]-HM-PAO as a tracer. The scanning procedure was performed 16-24 h after discontinuation of specific therapy. Tracer activity ratios were determined in 10 pairs of cerebellar, cortical, and subcortical regions. Data were compared with those of 10 age-matched controls. Most of the regions examined did not show any relevant change between parkinsonian and control subjects. Notably, mean activity in striatal regions were similar in the two groups. Increased activity in caudate-putamen was found in patients who were on chronic DOPA therapy. Side-to-side asymmetries in the basal ganglia increased with the severity of the disease. Significant reductions of tracer uptake, from control values, were observed bilaterally in the parietal cortex. These deficits were more pronounced in patients with mental deterioration and in subjects who had been chronically treated with anticholinergic drugs. Parietal perfusion deficits in parkinsonian patients resemble those described in Alzheimer's dementia. These findings suggest that the heterogeneous alterations of regional cerebral blood flow (rCBF) in parkinsonian patients reflect the multifactorial pathophysiology of the disease.