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This study attempted to establish normal values for cerebral white and deep gray matter density and total brain density, and to discover how much dilatation of the cerebrospinal fluid--containing spaces occurs with advancing age up to 40 years. The 53 female and 62 male healthy volunteers, 15-40 years old, had been screened to exclude individuals with neurologic disease, previous head trauma, congenital or acquired heart disease, chronic pulmonary disease and other systemic illness, and those who consumed more than small amounts of alcoholic beverages. The computed tomography scan data for the 115 subjects were scored subjectively for the severity of atrophy. It was found that in both genders there was an increasing frequency with advancing age of sulcal widening of the frontal lobes and cerebellar vermis starting in the teens. A ventriculo-internal cranial ratio was calculated for 93 subjects who had been examined on the EMI CT 1010, the mean value being 0.31 +/- 0.08 for females and 0.33 +/- 0.06 for males. The ratio did not change significantly with age up to 40 years. On the same 93 subjects, mean values and standard deviations were obtained for normal white matter (30.1 +/- 3.5 Hounsfield units (H) for females and 29.8 +/- 3.3 H for males) and for normal deep gray matter (33.0 +/- 3.3 H for females; 33.2 +/- 2.6 H for males) and for total brain density (33.9 +/- 2.7 H for females; 33.6 +/- 2.6 H for males).