We investigated the correlation between electroencephalography (EEG) abnormalities and dementia in Parkinson's disease. Three groups of subjects were examined: Parkinson's disease patients without dementia, Parkinson's disease patients with dementia, and healthy age-matched controls. Dementia was assessed with the use of Mini-Mental State Examination. All subjects underwent a routine EEG examination with determination of arousal reaction to eye opening, hyperventilation and photic stimulation. Demented patients with Parkinson's disease more often showed no response to eye opening and presented more slow wave activity in comparison to non-demented patients and healthy controls.