Two patients had delayed development and generalized muscle hypotonia and weakness since early infancy. Their muscle biopsies showed slight variation in fiber size without group atrophy, and no clear evidence of an active demyelinating process in the intramuscular nerves. The most striking finding by light microscopy was the absence of myelinated fibers in the intramuscular nerve bundles. Ultrastructurally, the axons were devoid of myelin sheath or had very thin myelin sheaths, and the axons were surrounded by multilayered basal lamina forming an atypical onion bulb with no suggestions of myelin destruction. A sural nerve biopsy from one of the patients showed similar findings. Unlike the Déjèrine-Sottas type of hereditary motor and sensory neuropathy, there was no evidence of demyelination and remyelination in the muscle pathology, suggesting that the poor myelination in congenital hypomyelination neuropathy is due to a true "hypo"-myelination and not the result of demyelination.