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The P300 component of the auditory EEG event-related potential (ERP) and Positron Emission Tomography (PET) were investigated in individuals considered to be early cases of probable Alzheimer's type dementia (PAD) and a control group of similar age. Two questions were investigated: (a) The degree to which P300 latency was sensitive to neurocognitive changes in very early stages of PAD; and (b) The relationship between P300 latency and specific areas of reduced neural functions as reflected in PET scans. A significant difference was found between PAD and normal subjects in P300 latencies, with those in the patient group having longer P300 latencies than the control group. The P300 latencies were significantly correlated with cortical, but not subcortical, metabolic rates. The highest correlation was between P300 latency and pareital PET scores, with other significant correlations with P300 latency in the following order: frontal, temporal and parahippocampal. These data indicate that P300 latency is an index of the integrity of association cortex and, as such, is sensitive to the very early stages of Alzheimer's dementia.