We assessed axonal loss in the normal appearing white matter of the corpus callosum in postmortem brains of patients with multiple sclerosis, using quantitative measures of both axonal density and white matter atrophy. The calculated total number of axons was reduced significantly (mean +/- SD, 5.4 x 10(7) +/- 3.1 x 10(7)) compared with normal controls (11.6 x 10(7) +/- 2.2 x 10(7), p = 0.001) with a reduction both in axonal density (median, 34%; range, 16-56%; p = 0.004) and area (mean +/- SD: multiple sclerosis, 584 +/- 170 mm2; controls, 871 +/- 163 mm2; p = 0.004). These results confirm substantial axonal loss in the normal appearing white matter and demonstrate that measures of both axonal density and white matter volume are necessary to appreciate the full extent of axonal loss.