Apolipoprotein E (ApoE) epsilon 4 allele is a risk factor for late-onset Alzheimer's disease (AD) and proposed to have a direct impact on cholinergic function in AD. Slowing of the EEG is characteristic in AD and the cholinergic system has an important role in modulating EEG. Fifty-eight AD patients at the early stage of the disease and 34 age- and sex-matched controls were studied using the quantitative EEG recorded from T6-O2 derivation. AD patients were divided into two subgroups: a) according to the ApoE allele (2 epsilon, 1 epsilon 4, and 0 epsilon 4) and b) according to familial aggregation. AD subgroups did not differ in clinical severity or duration of dementia. The AD patients with the epsilon 4 allele had higher relative theta amplitude and lower relative beta amplitude than controls and patients without the epsilon 4 allele. The peak frequencies were lower in all AD subgroups compared to controls. In conclusion, we found a tendency towards more pronounced EEG slowing in AD patients carrying the epsilon 4 allele. The minor differences in EEG may suggest differences in the degree of the cholinergic deficit in these subgroups.