In the present pilot study, age-related white matter changes were investigated by the use of design-based stereological methods. In the brains of elderly subjects, the total volume of the white matter and the total volume of the myelinated fibers therein were lower than in those of young subjects (15% and 17%, respectively), but the differences were not statistically significant. The total length of the myelinated fibers of the white matter in the elderly group of 86,000 km was, statistically, significantly decreased by 27% compared with 118,000 km in the young group. This loss of the total nerve fiber length was accompanied in particular by a decline of the myelinated fibers with a small diameter. The mean diameter of the myelinated fibers in the young group was significantly smaller than in the old group, but the relative size distributions of the myelinated fiber diameters between the young and old groups were similar. Our findings show that the atrophy of the human white matter during ageing is probably caused by a loss of myelinated fibers with a small diameter.