Recent studies have indicated that early brain injury may be responsible for the detrimental effects seen in patients after subarachnoid hemorrhage (SAH). In this study, we investigated the relationship between apolipoprotein E gene (APOE) polymorphism and the change of brain function in the early stage of aneurysmal SAH. A total of 79 patients admitted within 5 days after aneurysmal SAH were recruited in the study. Patient characteristics, such as age, gender, Fisher and Hunt-Hess grade were collected when admitted. Electroencephalogram (EEG) was recorded on admission and at 3-5 days after onset to assess the change of brain function of the patients in acute stage of SAH. The result of the second EEG recording was defined as EEG deterioration if the decrease in alpha wave frequency, increase in slow wave or decline in amplitude were observed when compared with the first EEG recording. The APOE polymorphism was determined in all patients by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Ten of 17 patients with APOEε4 (58.8%) showed the deteriorated EEGs, which was significantly different from those without APOEε4 (18 of 62 patients, 29.0%, p=0.023). However, neither the presence of ε2 nor of ε3 was significantly different from those absent of it (p>0.05). Univariate logistic regression analyses showed that both high Fisher grade (p=0.028, OR=2.917, 95% CI=1.124-7.572) and APOEε4 (p=0.027, OR=3.492, 95% CI=1.150-10.604) were risk factors to EEG aggravation after aneurysmal SAH. The association of APOEε4 for deteriorated EEG was more significant after adjustment for age, gender, Hunt-Hess grade on admission, and Fisher grade (p=0.007, OR=5.741, 95% CI=1.625-20.280). Our findings suggest that APOEε4 allele is a risk factor to brain function aggravation in the early stage of aneurysmal SAH, and it may contribute to early brain injury after SAH.