In Parkinson's disease patients with cognitive deterioration, regional cortical hypometabolism has been observed with [¹⁸F]fluorodeoxyglucose‐positron emission tomography (FDG‐PET). Our aim was to develop a robust method to subsume the overall degree of metabolic deterioration in Parkinson's disease by means of a single index and to investigate which of the clinical features correlates best with hypometabolism. Twenty‐two Parkinson's patients (10 demented) and seven controls underwent FDG‐PET. A metabolic index (mean relative uptake in typically affected regions) was calculated for each patient and compared with scores for cognition [Minimental State Examination (MMSE)], motor performance [Unified Parkinson's Disease Rating Scale (UPDRS III)” and behavior (Neuropsychiatric Inventory). In stepwise linear regression analysis, MMSE (P < 0.001) score showed the only significant effect. Estimated sensitivity and specificity for DSM‐IV diagnosis of dementia were high for the metabolic index (MI), with 91 and 100%. Taken together, the presented data indicate that cerebral hypometabolism in Parkinson's disease is primarily associated with cognitive impairment. © 2009 Movement Disorder Society