In order to investigate an influence of advancing age on regional cerebral blood [flow](rCBF), 125 measurements of rCBF were made by 133Xe inhalation method in 90 normal subjects without cerebral symptoms and risk factors of atherothrombotic stroke including hypertension, diabetes mellitus, and hyperlipidemia. There were 46 male and 44 female aged 19 to 80 years old (mean 42.5). The measurements were carried out in a quiet, semi-darkened room with the subjects at rest and eye closed. 133Xe inhalation system (Meditronic-Novo Diagnostic Systems, Inhalation Cerebrograph, Denmark) consists of 32 scintillation detectors, the lateral part of each hemisphere being covered by 16 detectors placed in parallel. One pair of the detectors is mounted in place over the brain stem and cerebellar region. Relationship of age and rCBF computed as the initial slope index (ISI) by Fourier analysis was investigated with both correlation and regression analyses, and age-matched normal values were calculated. End-tidal partial pressure for carbon dioxide (PECO2) was recorded from the face mask during the measurements along with the mean arterial blood pressure (MABP). ISI values were not corrected for changes in PECO2. PECO2 and MABP did not differ significantly among the normal subjects. There were no significant differences in the hemispheric mean ISI values between the right and left hemispheres. Mean brain ISI values showed significant negative correlation with advancing age (r = -0.67, p less than 0.001), and regression line was Y = -0.30X + 67.8. Ninety five per-cent confidence interval of the regression line was +/- 11.8. Regional ISI values also showed significant negative correlations with advancing age in the whole brain (p less than 0.001). There were no significant differences in the correlation coefficients obtained from parallel regions between the right and left hemispheres. The regional reduction of ISI values with advancing age was significantly greater in the regional distribution of the middle cerebral arteries bilaterally, compared with regions in the distribution of either the anterior cerebral, the posterior cerebral, or vertebrobasilar arteries (p less than 0.05). Regional hemispheric percent value of frontal lobe was greater than that of either parietal, temporal, occipital lobe, or brain stem and cerebellar region in the whole age. But, this hyperfrontal distribution became gradually obscured with advancing age, because reduction of ISI values with aging was significantly greater in the frontal than in other regions. It is concluded that measured rCBF of a patient have to be compared with age-matched normal values of mean brain and each region. Judgment image was produced with dividing patient value by 95% confidence limit of age-matched normal value. According to this image, regions in which rCBF significantly decrease or increase can be easily evaluated.