Cognitive deficits are common in Parkinson's disease (PD), but the pathophysiology and relationship to Alzheimer's disease (AD) are not understood. We used a case-control format to investigate putative risk factors for the development of dementia in patients with Parkinson's disease. We compared 52 cognitively intact patients with PD to 43 PD patients with dementia with regard to factors previously suggested as relevant to either AD or PD. Multiple logistic regression yielded the following significant predictors of dementia in PD: lack of education (less than a high school graduate) (OR 21); severity of motor deficit (UPDRS total motor score greater than 20; OR 6.34), and PD onset at greater than 60 years of age (OR 4.12). The predictive probability of dementia in our subjects when all three variables were positive was 97.9%. We conclude that education may modify the risk of cognitive decline in PD. Protective effects of educational attainment, independent of dementia etiology, may be due to greater functional brain reserve.