The peripheral neuropathies constitute approximately 30% of cases seen in a pediatric neuromuscular service. Two-thirds of such cases are chronic. Of the latter group roughly 70% are hereditary, 20% indeterminate and 10% acquired. There are definite chromosomal localizations for an increasing number of the hereditary neuropathies: to the region of the centromere of the X chromosome in an X-linked form of Charcot-Marie-Tooth (CMT) disease, to chromosome 17 in the commonest form of CMT, and to the long arm of chromosome 1, close to the Duffy locus, in a few kindreds with a rarer type of CMT. A duplication of DNA on the long arm of chromosome 17 is the commonest abnormality in autosomal dominant CMT. The duplicated region contains the gene for a 22 kDa myelin protein, PMP-22. The gene for PMP-22 is the site of a mutation in the hereditary neuropathy of the Trembler mouse. Some sporadic and inherited cases of chronic hypertrophic demyelinating neuropathy have recently been shown to be associated with specific mutations of the myelin protein P0 and PMP-22 loci. By contrast, a deletion encompassing the PMP-22 locus has been found in tomaculous neuropathy (hereditary liability to pressure palsies). Finally, mutations of the gap junction protein, connexin 32, have been shown to be responsible for X-linked dominant CMT.