Intellectual abnormalities are common in Parkinson disease (PD), occurring in a majority of patients and exhibiting a spectrum of severity from mild to severe. Alzheimer disease (AD) has been posited as the cause of dementia in PD. Comparative neuropsychological studies, however, show differences in memory, language, and frontal lobe functions between AD and PD patients even when the two groups have comparably severe dementia syndromes. The AD-type neuropathology occurs in 10–60% of PD patients, and dementia is usually overt when AD pathology is identified at autopsy. The AD changes are less frequent than intellecutal deterioration in PD, and dementia has been observed in PD patients without AD pathology. Therefore, concurrent AD cannot be the cause of all cases of dementia in PD. Cholinergic deficits occur in some PD patients, but cholinergic deficits have been described in patients without dementia and dementia has been documented in patients without cholinergic system abnormalities. Dopaminergic disturbances contribute to the dementia of PD. Differences in neuropeptide concentrations, electrophysiologic responses, and cerebral metabolism also support patholphysiologic distinctions between AD and the dementia of PD. Genetic investigations suggest a role for heredity in AD, whereas PD appears to be an acquired, nongenetic disorder. These studies indicate that despite areas of overlap in clinical symptoms and neuropathology, AD and the dementia of PD are largely distinct.