Observations have been made on the peripheral nerve changes in four patients, ranging in age from 4 to 32 years, with the congenital cataracts facial dysmorphism neuropathy syndrome. Myelinated fibre density was within normal limits. The salient abnormality was diffuse hypomyelination which, in the older patients, was associated with demyelination and then axonal degeneration. These findings could be correlated with the relative preservation of sensory action potential amplitude despite markedly reduced nerve conduction velocity. Unmyelinated axon density was preserved. The morphological observations suggest the operation of a developmental process affecting myelination with a later superimposed degenerative disorder.