Brain atrophy in 228 normal persons was quantitatively measured by computed tomography (CT), making use of computer programs available in the CT system. Pixels representing bone, brain, and cerebrospinal fluid in slices thru the head were counted by the computer. The volume percentage of the brain to the cranial cavity (Craniocerebral Index: CCI) was calculated from the pixel numbers. The CCI in 20 to 49-year-old adults was constant at 92 ± 3. This value was considered the standard of normal brain and the percentage of the subject's CCI to this value (Brain Volume Index: BVI) was calculated as the indicator of brain atrophy. The BVI was rather low in children (97 ± 5) and teenagers (96 ± 3), and decreased drastically with increasing age after 50 years. The BVI in the 80s was as low as 89 ± 6.