During mouthpiece respiration of Xe-133 for a measurement of regional cerebral blood flow (rCBF), the breathing pattern of patients fluctuated and it caused a change of end-tidal CO2 concentration that had an excellent correlation with PaCO2 in patient without respiratory disease. The end-tidal CO2 concentration of demented patients varied within lower ranges than senile control group. The range of fluctuation on the end-tidal CO2 concentration was dependent on the type and the degree of dementia, and it fluctuated most widely at the middle stage of Alzheimer disease. Mean cerebral blood flow increased by 13.9% for each 1% increase in end-tidal CO2 concentration (3.6%/mmHg PaCO2) in the case of demented patients without cerebrovascular disease. To improve the reliability of rCBF in demented patients, especially in Alzheimer disease, the correction of rCBF data for end-tidal CO2 concentration should be performed.