Identification of preclinical markers is required for early diagnosis of Alzheimer's disease (AD) and cognitive dysfunction in advancing age. Quantitative EEG was examined in 145 individuals with AD, their unaffected relatives and unrelated individuals. The AD patients and their relatives were stratified by ApoE genotype. The resting EEG parameters were severely changed in AD patients, and in patients carrying the ApoE epsilon4 allele the decrease in alpha power was higher than in epsilon4 non-carriers. The resting EEG parameters were indistinguishable in AD relatives with different ApoE genotypes and similar to EEG pattern in common population. Under hyperventilation the presence of the epsilon4 allele in AD relatives was associated with the manifestation of synchronous high-voltage delta-, theta-activity and sharp-waves, pronounced decrease in alpha and increase in delta and theta relative powers. The data suggest that neurophysiological endophenotype of non-demented individuals at genetic risk for AD, characterized by increased excitability and dysfunction of deep brain and alpha rhythm-generating structures, may be revealed decades before the first clinical symptoms of presumable dementia.