Congenital demyelinating motor and sensory neuropathy with focally folded myelin sheaths. Six patients (5 index cases and 1 sib) with a congenital motor and sensory neuropathy are described. The clinical, genetic and electrophysiological features resembled Dejerine-Sottas disease or hereditary motor and sensory neuropathy (HMSN) type III. Sural nerve biopsy of 5 patients revealed segmental demyelination and remyelination with hypertrophic changes, although onion bulbs were not as ubiquitous as in classical HMSN type III. A striking discriminating feature from HMSN type III was an abundance of focal myelin thickenings (tomacula) present in nearly all teased fibres. Possible pathogenic implications are discussed. These cases corroborate the heterogeneity of congenital motor and sensory neuropathies. 