A patient with congenital insensitivity to pain with anhidrosis, who had characteristic clinical features and biopsied sural nerve, is presented. Nerve pathology findings indicated a loss of the small myelinated and unmyelinated fibers. Biopsied muscle disclosed a marked variation in fiber size, some small fibers with central nuclei, and a small number of small angulated fibers, consistent with neurogenic and myogenic changes. Many patients with congenital insensitivity to pain with anhidrosis had muscle weakness and absent or decreased deep tendon reflexes with normal nerve conduction velocity. We confirmed that lack of small myelinated fibers in motor neurons resulted in a striking change of muscle in our patient.