The authors examined whether the ±4 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 ±4 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 ±4 allele was identified using a restriction enzyme. The frequency of the ApoE ±4 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 ±4 allele frequency (16%) than age-matched controls (10%,p= 0.07). The ±4 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 ±4 allele is associated with dementia in PD.