Longitudinal changes in cerebral glucose metabolism in 46 patients with Parkinson’s disease (PD) were retrospectively analyzed. 18F-fluorodeoxyglucose positron emission tomography was undertaken both at baseline and after 3 years of follow-up. Three of 29 cognitively normal PD patients and 3 of 17 PD patients with mild cognitive impairment (PD-MCI) developed dementia during follow-up. Dementia converters showed parieto-occipital hypometabolism at baseline regardless of their initial cognitive status. A finding of parieto-occipital hypometabolism is useful in predicting dementia in PD.