Severe dementia affects 10 to 20% of all patients with Parkinson's disease (PD) and is particularly common in those aged 65 years and over. In a clinicopathologic study, we correlated Mini-Mental State Examination scores and DSM-III dementia ratings with the density of Lewy bodies, Lewy neurites, neurofibrillary tangles, neuritic plaques, gliosis, and neurons in the hippocampus and amygdala of 27 PD patients without Alzheimer's disease changes. Cortical Lewy body densities were examined in the anterior cingulate gyrus. The degree of cognitive impairment was correlated with the density of Lewy neurites in the CA2 hippocampal field, raising the possibility that disruption of the connection between the dentate gyrus, entorhinal cortex, septal nuclei, and hypothalamus and the CA1 field contributes to dementia in PD.