The authors examined whether the epsilon4 allele might be associated with dementia in Parkinson disease (PD), given that the dementia of PD shares neuroanatomic and neurochemical features with Alzheimer disease (AD) and that many recent studies have found a high prevalence of the epsilon4 allele of apolipoprotein E (ApoE) in AD. The authors examined patients with PD (n=125, 47 demented) and unrelated controls (n=93) using a short mental test. DNA was obtained from blood leukocytes. The relevant portion of the apolipoprotein E (ApoE) gene was amplified by polymerase chain reaction, and the epsilon4 allele was identified using a restriction enzyme. The frequency of the ApoE epsilon4 allele in demented patients with PD (14%) was not greater than that in nondemented patients (17%), whereas patients with PD as a whole showed a trend toward a higher epsilon4 allele frequency (16%) than age-matched controls (10%, p=0.07). The epsilon4 allele frequency in nondemented patients with PD was significantly higher than in controls (p=0.055). These results and the meta-analysis of four published reports fail to support the hypothesis that the epsilon4 allele is associated with dementia in PD.