A computerized EEG (cEEG) study was performed in two groups of patients with dementia of the Alzheimer type (DAT) and with Parkinson's disease with dementia (PDD). Compared to control subjects suffering from Parkinson's disease without dementia, both groups demonstrated similar slowing down of background activity. However, there were more low frequency components, defined as between 2.5-7 Hz, and less topographical differentiation in PDD than in DAT. Furthermore, more severe impairment of cognitive functions was observed in DAT, but their psychometric score correlated with slowing down of EEG only in DMP. These observations emphasize the influence of the subcortical/cortical axis both in dementia of Parkinson's disease and in the neurophysiology of the EEG.