We analyzed 136 patients (age, 67.5 ± 6.9 years; men, 59.6%) with idiopathic rapid eye movement sleep behavior disorder (iRBD). The results of the neuropsychological tests were not significantly different between APOE ε4 carriers and noncarriers, suggesting that the APOE ε4 allele was not associated with cognitive impairment in iRBD.