EEG power amplitude and power ratio data obtained from 15 (3 men and 12 women) patients with Alzheimer's disease (AD) and 8 (2 men and 6 women) with senile dementia of Alzheimer type (SDAT) were compared with similar data from 40 age- and sex-matched normal controls. Compared with the healthy controls, both patient groups demonstrated increased EEG background slowing, and it indicated more slower in AD than in SDAT. Moreover, both groups showed characteristic findings respectively on EEG topography and t-statistic significance probability mapping (SPM). The differences between AD and their controls indicated high slowing with reductions in alpha 2, beta 1 and beta 2 activity. The SPMs of power ratio in theta and alpha 2 bands showed most prominent significance in the right posterior-temporal region and delta and beta bands did in the frontal region. Severe AD indicated only frontal delta slowing compared to mild AD. The differences between SDAT and their controls indicated only mild slowing in delta and theta bands. The SPM of power amplitude showed occipital slowing, whereas the SPM of power ratio showed the slowing in the frontal region. Judging from both topographic findings, these were considered to denote diffuse slow tendency. In summary, these results presumed that in AD, cortical damages followed by EEG slowing with reductions of alpha 2 and beta bands originated rapidly and thereafter developed subcortical (non-specific area in thalamus) changes with frontal delta activity on SPM. On the other hand, in SDAT, diffuse cortico-subcortical damages with diffuse slowing on EEG topography were caused gradually.