Gender differences in age effect on brain atrophy measured by magnetic resonance imaging. A prospective sample of 69 healthy adults, age range 18-80 years, was studied with magnetic resonance imaging scans (T2 weighted, 5 mm thick) of the entire cranium. Volumes were obtained by a segmentation algorithm that uses proton density and T2 pixel values to correct field inhomogeneities ("shading"). Average (+/- SD) brain volume, excluding cerebellum, was 1090.91 ml (+/- 114.30; range, 822.19-1363.66), and cerebrospinal fluid (CSF) volume was 127.91 ml (+/- 57.62; range, 34.00-297.02). Brain volume was higher (by 5 ml) in the right hemisphere (P less than 0.0001). Men (n = 34) had 91 ml higher brain and 20 ml higher CSF volume than women (n = 35). Age was negatively correlated with brain volume [r(67) = -0.32, P less than 0.01] and positively correlated with CSF volume (r = 0.74, P less than 0.0001). The slope of the regression line with age for CSF was steeper for men than women (P = 0.03). This difference in slopes was significant for sulcal (P less than 0.0001), but not ventricular, CSF. The greatest amount of atrophy in elderly men was in the left hemisphere, whereas in women age effects were symmetric. The findings may point to neuroanatomic substrates of hemispheric specialization and gender differences in age-related changes in brain function. They suggest that women are less vulnerable to age-related changes in mental abilities, whereas men are particularly susceptible to aging effects on left hemispheric functions. 