Abstract In a Japanese nationwide survey on the use of FDG-PET for dementia, a total of 406 cases acquired during the period from April 2001 to March 2002 were collected from 15 PET centers. The purpose of the PET scans included (A) diagnosis of patients with memory disturbance for possible early stage of Alzheimer's disease (154 cases) and (B) differential diagnosis for dementia-related neurodegenerative diseases (144 cases), and the PET achieved the purpose in most cases. Based on the survey, an economic evaluation was performed on the 2-year projected cost and benefit of treating patients with donepezil that are diagnosed as Alzheimer's disease with FDG-PET. The estimation indicated that early and/or accurate diagnosis of Alzheimer's disease with FDG-PET followed by initiation of the donepezil treatment would delay the disease progress and reduce the care cost more than it would increase the cost by the PET and the drug therapy. The average reduction of the total cost was estimated to be ¥61 500 (A) and ¥13 700 (B) per person. The increase of the quality adjusted life year (QALY) was predicted to be 0.044 (A) and 0.014 (B). Therefore, incorporation of PET into the clinical path was shown to be economically dominant.