The epsilon 4 isoform of apolipoprotein E (Apo-E) may confer genetic susceptibility for familial and sporadic Alzheimer's disease (AD). Because dementia in AD and Parkinson's disease (PD) share many biologic and clinical features, we determined the Apo-E genotypes for 79 patients with PD, 22 of whom were demented, and for 44 age-matched healthy elderly controls from the same community. We hypothesized that if the dementia was similar to AD, there would be a higher allele frequency of apolipoprotein epsilon 4 (Apo epsilon 4) in demented PD patients compared with nondemented PD patients and controls. The epsilon 4 allele frequency for PD without dementia was 0.132, for PD with dementia, 0.068, and for controls, 0.102. There was no association between Apo epsilon 4 and dementia in the PD patients. We conclude that the biologic basis for dementia in PD may differ from that of AD.