1. A clinical, genetic, electrophysiological and nerve biopsy study of 49 index cases with peroneal muscular atrophy is reported. 2. In dominantly inherited cases, motor conduction velocities of the upper limbs within kinships indicated segregation into five groups which we have termed: a) hypertrophic neuropathy (less than 25 m/sec); b) intermediate group (25-45 m/sec); c) neuronal sensorimotor neuropathy (greater than 45 m/sec); d) neuronal motor neuropathy (greater than 45 m/sec); e) neuronal motor neuropathy with upper motor neurone involvement (greater than 45 m/sec). 3. The intermediate group is distinguished from the hypertrophic neuropathy group by the absence of clinically observed nerve hypertrophy and by the presence of a number of clinical features, including a more rapidly progressive disease. It is concluded to be genetically separate. This group is similarly quite distinct from the neuronal groups. Nerve biopsy studies support this view (Madrid et al., 1977; Bradley et al., 1977). 4. There was a relationship between severity of the disease and the conduction velocity which was most evident in the intermediate group. 5. There appeared to be an increase in motor conduction velocity with age in the hypertrophic neuropathy group, while the velocity fell in older patients in the intermediate group. 6. Sensory conduction velocity generally paralleled motor velocity but showed relatively less reduction. 7. Upper motor neurone features were not uncommon, appearing particularly in the intermediate group. Their presence may not therefore be a reliable basis for the classification of cases of peroneal muscular atrophy. 8. Tremor was observed in the hypertrophic, intermediate and neuronal motor neuropathy groups of patients, and does not provide a useful criterion for the classification of peroneal muscular atrophy. 9. There was occasional evidence to suggest poor or abnormal expression of the gene in dominantly inherited cases. Until more specific markers are available sporadic cases should be classified on the basis of the dominant forms, though they show a greater variability.