In a typical case of Cockayne's syndrome, morphometric histopathologic findings of the sural nerve were described and compared with data on two controls. The main pathologic changes of myelinated fibers were segmental demyelination and remyelination with onion bulb formation and a moderate decrease of myelinated fibers. There was no evidence of axonal atrophy in relation to myelin spiral length. In addition, no evidence of the clustered occurrence of demyelinated and remyelinated internodes was obtained on the statistical evaluation of the distribution of such internodes. Although the presence or absence of hypomyelination of a mild degree and maldevelopment of axons to be myelinated has not been clarified, our morphometric data do suggest the presence of primary segmental demyelination of the biopsied sural nerve.