Design.
—Case series, group comparisons.

Setting.
—Ambulatory care referral center.

Patients.
—Consecutive sample of 14 patients with PD dementia and 27 with probable AD matched for overall intellectual function using a mental status test, as well as 1 nondemented PD and 12 mild probable AD patients, similarly matched for overall intellectual function. All demented patients met Diagnostic and Statistical Manual, Revised Third Edition, criteria for dementia.

Main Outcome Measures.
—Performance on a battery of neuropsychological tests assessing verbal and nonverbal memory, verbal fluency, and constructional ability.

Results.
—Nondemented and demented patients with PD performed worse than their probable AD comparison groups on verbal fluency and visuospatial tasks. Cognitive changes attributable to dementia were similar in PD and probable AD but were not identical. The patients with probable AD demonstrated more marked change in memory performance with delay.

Conclusions.
—Our findings suggest that when dementia occurs in PD it is overlaid on cognitive changes that already exist in nondemented patients but that the dementing process in PD involves systems other than those responsible for cognitive change in nondemented PD patients. We hypothesize that in most cases, dementia in PD involves changes in a nondopaminergic neurotransmitter system but is not due to concomitant AD.