Biopsy of the sural nerves distinguished two groups of patients with peroneal muscular atrophy (Charcot-Marie-Tooth): a hypertrophic type and a neuronal type. In patients with the hypertrophic type (10 nerves), 30-100 per cent of teased fibres of the sural nerve had demyelinated segments, numerous onion-bulb formations and often an increase in endoneurial space. Large and small fibres, with a diameter of more than 7 micron and less than 5 micron were diminished in number. Regeneration was scarce. There were more fibres with 60-120 myelin lamellae than in normal nerve, suggesting an atrophy of the axon. Biopsy of 19 sural nerves of patients with the neuronal type of PMA showed loss of large fibres (more than 7 micron in diameter). The number of small fibres was normal, presumably due to regeneration, since there were many "clusters" of small myelinated fibres. Fibres with demyelinated segments and onion-bulb formations were absent or rare and the endoneurial space was normal or slightly increased. Neither in the hypertrophic nor in the neuronal type did fibre loss occur selectively among the very largest fibres. Nine nerves from patients with hereditary spastic paraplegia and from a family with tremor and spasticity in addition to PMA showed changes similar in type but often milder in degree than nerves of the neuronal type of PMA. The number of unmyelinated fibres was normal in 12 of 21 nerves from patients with PMA; it was increased in 5 and diminished in 3 nerves.