EEG frequency analysis by Fast Fourier Transform (FFT) was studied in different stages of Alzheimer's disease (AD), defined according to the neuropsychological test score. It was observed that in mild AD the percentage power of the theta band, the ratio of powers in the alpha and theta bands and the mean frequency (range 1.46-20.02 c/sec) differed significantly from the old healthy controls. The percentage power of the alpha band, the occipital peak frequency and the ratio of powers in the alpha and delta bands decreased linearly in different stages of AD. These variables did not differ significantly in mild AD from the control values. Distinct slowing of the occipital peak frequency and distinct accentuation of the percentage power of the delta band occurred in advanced AD. Slowing of the dominant occipital rhythm and accentuation of the diffuse irregular slow waves, which are usually regarded as the main EEG criteria for AD, do not describe mild but advanced disease.