We investigated two patients with hereditary motor and sensory neuropathy type III, one with Déjérine-Sottas disease and the other with congenital hypomyelination neuropathy based on nerve pathology and MRI of the sciatic nerve. On biopsy of the sural nerve of the patient with Déjérine-Sottas disease, myelin debris, indicating demyelination, was observed in an onion-bulb pattern surrounding myelinated fibres. In the patient with congenital hypomyelination neuropathy, onion bulbs were formed of two parallel layers of basement membrane. There was no evidence of myelin breakdown. On axial T2-weighted MRI, a severely hypertropied sciatic nerve containing multiple rounded lesions, suggesting inflammation or demyelination, was observed in the patient with Déjérine-Sottas disease. In contrast, the sciatic nerve of the patient with congenital hypomyelination neuropathy showed slight hypertrophy without demyelination. MRI of the sciatic nerve may represent a useful tool for characterisation of demyelinating disease and its prognosis.