With magnetic resonance (MR) imaging, brain water self-diffusion was measured in 17 healthy volunteers 22-76 (mean, 44.6) years old. The calculated values for the apparent diffusion coefficients (ADCs) ranged from 0.58 x 10(-9) to 1.23 x 10(-9) m2/sec in cerebral white matter. A significant correlation was found between the ADC in white matter and age (r = .7069, P < .01). The calculated values for ADC in cortical gray matter ranged from 1.06 x 10(-9) to 1.72 x 10(-9) m2/sec; no correlation was found between ADCs in gray matter and age. The increased ADC in white matter may be caused by an increase in the extracellular volume due to age-dependent neuronal degeneration or to changes in myelination. These findings have implications for future clinical investigations with diffusion MR imaging techniques in patients with neurologic diseases, and stress the importance of having an age-matched group of healthy volunteers for comparison.