Abstract: Dementing illnesses comprise Alzheimer's disease (AD), Pick's disease, Multi-infarct dementia (MID) and other neurological disorders. These diseases have different clinical characters respectively. Neuropsychological examinations can help to diagnose and differential diagnose dementias. The development of neuroimaging dementias is more and more rapid. ¹⁸F-FDG PET method shows neocortical hypometabolism occurring in the biparietotemporal lobes and left-right asymmetry of AD patients in the early stage . It can also differential diagnose AD from other dementias.