Sural nerves were examined in 3 childhood cases of Leigh's disease (from 2 families), each with electrophysiological documentation of peripheral neuropathy. Postmortem confirmation of Leigh's disease was made in 2 cases; the third had characteristic CT scan appearances. Myelinated fibre densities were within normal limits, with no evidence of degeneration or regeneration. The myelin sheaths, however, appeared thin and this was confirmed quantitatively. In teased fibres there was no demyelination in 2 cases but some in 1 case. The findings suggest that there is hypomyelination of peripheral nerves leading eventually to demyelination.