A three year-old boy with congenital sensory neuropathy with anhidrosis (CSNA) was described. Sural nerve biopsy specimens revealed an almost complete absence of unmyelinated fibers and a marked decrease of the density of small myelinated fibers with preservation of the density of large myelinated fibers. No evidence of active degeneration of unmyelinated or myelinated fibers was found. Skin biopsy specimens revealed the absence of nerve terminals and fibers innervating sweat glands, although sweat glands seemed to be apparently normal in their morphological findings. Therefore, it was concluded that the absence of pain and temperature sensations with preservation of touch sensation in our patient was compatible with the morphometric findings of nerve fibers of the sural nerve described. Similarly anhidrosis was concluded to be well explained by the absence of the innervation of sweat glands and the vessels around them. On the other hand, electrophysiologic studies, such as motor and sensory nerve conduction, short latency somatosensory evoked potential and auditory brainstem response, in which the function of the large myelinated fibers is presumably tested, were all normal. Therefore, the structure and function of such large myelinated fibers were spared in this case. From clinical viewpoints, electrophysiologic studies described above are useful to differentiate CSNA from other types of congenital sensory neuropathies, in which large myelinated fibers are affected.