Recent work from our laboratory demonstrated that quantitative electroencephalographic (EEG) coherence between brain areas linked by long cortico-cortical fibers (termed "fascicle" coherence) was differentially reduced in subjects with Alzheimer's disease, whereas coherence between brain areas linked by short cortico-cortical and cortico-subcortical fibers in postcentral areas (termed "visual" coherence) was differentially reduced in subjects with multi-infarct dementia. In this study, we investigated whether these differences in coherence represent "trait" or "state" markers for dementia. Visual coherence demonstrated high stability in both demented groups as assessed by both one-year test-retest reliabilities and analysis of group mean change. Fascicle coherence demonstrated good stability in multi-infarct dementia and control subjects, but some variability was observed in Alzheimer's subjects, suggesting both state and trait factors may be involved. These findings complement neuropathologic studies, and suggest that decreases in coherence may serve as a diagnostic trait markers for these two types of dementia. The role of state factors in Alzheimer's disease requires further investigation.