Regional spectral analyses of the EEG during wakefulness and REM sleep was performed in eight patients with mild to moderate Alzheimer's disease and compared to that of healthy age-matched controls. Alzheimer patients presented some EEG slowing in frontal, parieto-occipital and temporal regions in the waking state. However, this EEG slowing was much better revealed in REM sleep, whether expressed by the ratio of slow to fast frequencies or by changes in each of the frequency bands. Moreover, regional differences in EEG slowing, relatively nonexistent in the awake EEG, were a prominent feature of the REM sleep EEG. Importance of cholinergic basal forebrain in the desynchronization of the REM sleep EEG might explain these results because this structure is affected in early Alzheimer's disease.