<p>Brain metabolism is reduced in patients with dementia disorders, as demonstrated by hypometabolism on 2-deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]FDG) positron emissions tomography. A contributing factor to the hypometabolism could be decreased cerebral blood flow (CBF) leading to a state of subtle hypoperfusion‐induced tissue hypoxia causing a reduced brain oxygen metabolism and consequently elevated brain lactate. In the current exploratory study, we investigated brain lactate, global and regional CBF, and global cerebral metabolic rate of oxygen (CMRO₂) in patients with Alzheimer’s disease (AD) and dementia with Lewy bodies (DLBs). We hypothesized that the patients demonstrate a state of tissue hypoxia with reduced CMRO₂ and elevated brain lactate concentration. Participants included 24 AD patients, 10 DLB patients, and 15 healthy controls. MR spectroscopy measured lactate in the precuneus and occipital lobe. Global CBF and venous oxygen saturation (for CMRO2 calculation) were assessed using phase-contrast and susceptibility-based oximetry MRI, respectively. Regional CBF was measured with ASL-MRI. We observed no significant difference in either brain lactate or CMRO₂ between groups. The regional CBF in precuneus was significantly lower in AD compared to HC; however, this hypoperfusion was not associated with a higher lactate concentration. The lack of a difference in CMRO₂ or lactate concentration between patients and controls suggests that the hypometabolism observed in patients with AD and DLB may reflect structural neurodegeneration and not a state of tissue hypoxia. The local decrease of CBF in precuneus in patients with AD may be due to a lower CBF demand due to neurodegeneration.</p>