Functional brain imaging using computer-analyzed electroencephalography was performed in 40 subjects: 15 with mild-to-moderate dementia of the Alzheimer's type (DAT), 13 with mild-to-moderate multi-infarct dementia (MID), and 12 age-matched controls. We examined three different parameters of brain electrical activity in these subjects: absolute slow-wave power, proportional power in all frequency bands, and ratios of high-frequency/low-frequency electrical activity (so-called “spectral ratios”). Spectral ratios were significantly more powerful in discriminating among groups than the other measures. Functional images using spectral ratios revealed that subjects with DAT have a characteristic left temporo-parietal defect which clearly distinguished them from subjects with MID and from control subjects. The severity of dementia was best assessed by examining absolute slow-wave power, which had the strongest linear correlation with mental status testing. Serial images from one subject with DAT over 3 years demonstrate both quantitative and qualitative shifts in slow-wave activity in the course of DAT. The study suggests that functional imaging may be more useful than either simple EEG or computer-analyzed EEG in assessing and diagnosing patients with suspected dementia.