Background: Few comparative studies exist of metabolic brain changes among neurodegenerative illnesses. We compared brain metabolic abnormalities in Alzheimer's disease (AD) and in Parkinson's disease with dementia (PDD) as measured by proton magnetic resonance spectroscopy (MRS). Methods: Twelve patients with idiopathic PDD, 22 patients with probable mild AD, and 61 healthy older controls underwent posterior cingulate MRS. Results: Patients with AD exhibited reduced N-acetyl aspartate (NAA)/creatine (Cr) (P < .05) and increased choline (Cho)/Cr (P < .05) and myo-inositol (mI)/Cr (P < .01) compared with controls. Patients with PDD exhibited reduced NAA/Cr (P < .05) and glutamate (Glu)/Cr (P < .01) compared with controls. There was reduced Glu/Cr in PDD compared with AD (P < .01). Conclusions: Patients with AD and patients with PDD exhibited distinct brain metabolic MRS profiles. Findings suggest that comparison of brain MRS profiles across dementias provides useful direction for future study. © 2008 The Alzheimer's Association.