EEG topographic correlates of the severity and type of Alzheimer-type dementias have been studied in 21 patients with senile dementia (SD) and 18 with Alzheimer's disease (AD) who were subdivided into smaller groups by MMSE grades for the severity of dementia, as well as in 15 mentally normal elderly persons. EEC maps in SD and AD patients differed from those in normal subjects in increased theta-delta EEG spectral power and suppressed alpha-band power that was more pronounced in the left hemisphere in AD than in SD patients. AD and SD patients had also different EEG topographical changes with intensified cognitive dysfunctions. The results suggest that there is a great cholinergic deficiency and cortical rather than subcortical impairments in patients with AD than in those with SD.