EEG studies of Parkinson's disease (PD) have shown that the incidence of EEG abnormalities is higher than in normal old individuals. The most common alteration in PD is generalized slowing of the EEG. We studied 18 patients with Parkinson dementia, 18 age-matched Parkinson patients without dementia and 20 controls. The absolute and relative amplitudes of delta, theta, alpha and beta bands and the peak and mean frequency were calculated from EEG spectra recorded from the T6-O2 derivation. All variables differed significantly in Parkinson dementia patients compared to controls. The most conspicuous finding was the increase of delta activity. Parkinsonian patients without dementia had more theta activity and the frequencies were slow compared to controls. We conclude that parkinsonian subgroups have distinct patterns of abnormality in EEG spectra: Parkinson patients with dementia have distinctly slower EEGs than patients without dementia.