Abstract: Fifty cases of dementia in the elderly were follwed-up by EEG for 2-24 times within 27 years. These results showed that in all patients with normal EEG revealed normal intelligence. Patients with slight disturbance of intelligence, their EEG abnormal rates were found 39%. 58% and 33% respectively in the group of vascular dementia, Alzheimer’s diseases and mixed dementia, when patients confirmty diagnosed as dementia, their EEG abnormal rates in hese three groups increased significantly. The results were 89%, 67% and 70% respectively, and he asymmetry ratio of the vascular dementia was higher than that of Alzheimer’s diseases. The decreasing of Alpha index（αI） is along with the severity of dementia. When αI equals to zero, the EEG in patients with severe dementia was observed generalized slow activity. We conclude hat sequential followup of EEG and αI are the feasible and valuable parameters for the early diagonosis of dementia and provide scien-tific basis of prognosis.