["Dementia is a common feature of Parkinson's disease (PD), but the neuropathological changes associated with the development of Parkinson's disease dementia (PDD) are only partially understood. Mitochondrial dysfunction is a hallmark of PD but has not been studied in PDD.", 'Molecular and biochemical approaches were used to study mitochondrial activity and quantity in postmortem prefrontal cortex tissue. Tissues from pathologically confirmed PD and PDD patients and from age-matched controls were used to analyze the activity of mitochondrial enzyme complex nicotinamide adenine dinucleotide:ubiquinone oxidoreductase, or complex I (the first enzyme in the mitochondrial respiratory chain), mitochondrial DNA levels, and the expression of mitochondrial proteins.', "Complex I activity was significantly decreased (27% reduction; analysis of variance with Tukey's post hoc test; P\u2009<\u20090.05) in PDD patients, and mitochondrial DNA levels were also significantly decreased (18% reduction; Kruskal-Wallis analysis of variance with Dunn's multiple comparison test; P\u2009<\u20090.05) in PDD patients compared with controls, but neither was significantly reduced in PD patients. Overall, mitochondrial biogenesis was unaffected in PD or PDD, because the expression of mitochondrial proteins in patients was similar to that in controls."] Is dementia in Parkinson 's disease associated with enhanced mitochondrial complex I deficiency?