Brain atrophy in 154 men and 147 women without neurologic deficits was quantitatively studied by means of computed tomography. The ages ranged from 20 to 79 years. The volume percentage of brain to cranial cavity (Craniocerebral Index, or CCI) was calculated by means of computer programs available in CT systems. The mean CCI in the 20-39 age group was constant at 98.4 per cent in men and 98.7 in women, and this was considered the standard for the CCI in subjects with nonatrophied brains. The percentage of each subject's CCI in relation to this standard in both sexes (Brain Volume Index, or BVI) was calculated as the indicator of brain atrophy. The normal value for BVI was therefore 100 per cent in both sexes. A clear difference was observed between men and women in the process of brain atrophy with increasing age. In men a significant reduction in BVI began in the sixth decade of life (P less than 0.01); thereafter, the decline was gradual and steady through the eighth decade (P less than 0.05). In women, a significant reduction in BVI began in the fifth decade (P less than 0.01) and remained relatively constant during the fifth and sixth decades but declined significantly again in the seventh and eighth decades. In both sexes, remarkable individual differences in brain atrophy were characteristic.