A series of elderly patients with dementia of Alzheimer type (AD), multi-infarct dementia (MID) and functional (non-organic) psychiatric illness (major depressive disorder) were selected by DSM III-R criteria and had the following investigations: a battery of cognitive tests, EEG with power and coherence spectral analyses of T4-T6, T3-T5, P4-O2, P3-O1 channels, visual evoked potential (flash and pattern reversal) and P300 recordings as well as single photon emission tomography (SPECT) using 99mTc HMPAO. Three subsets of patients were chosen on clinical and SPECT criteria. These were as follows: patients with a clinical diagnosis of AD and a SPECT rCBF pattern showing bilateral temporo-parietal perfusion deficits (AD type), patients with a clinical diagnosis of MID and a SPECT rCBF pattern showing single focal perfusion deficits or multiple areas of low perfusion in the cerebral cortex suggestive of ischaemic change (MID type SPECT picture) and functionally ill patients with normal rCBF (controls). The AD type group differed from the MID rCBF group in having significantly less alpha and more delta 2 (2- < 4 Hz) power. The latter had significantly lower alpha power than the controls. The 2 dementia groups with abnormal rCBF patterns did not differ in terms of coherence spectra or P300 latencies, but both had lower within and between hemisphere alpha coherence values and longer P300 latencies than the "controls" with normal rCBF. There were no group differences in the flash VEP P2-pattern reversal P100 latency difference values.