Nerve conduction studies were performed on 45 patients in 17 kinships with autosomal dominant Charcot-Marie-Tooth disease. Sural nerve biopsies were performed on 20 patients. The mean motor conduction velocities in median, ulnar and lateral popliteal nerves were 27.5m/sec, 25.9m/sec and 26.3m/sec respectively. Sensory conduction was impaired in all cases. The fastest motor conduction velocities were found in 5 members of 1 kinship who were found on sural nerve biopsy to have the neuronal form of the disease in which there was a reduction in density of large diameter myelinated fibres. In this group segmental demyelination was not a prominent feature in teased fibre preparations and onion bulb formations were not seen on electron microscopy. In the other patients conduction velocities were more severely slowed and hypertrophic changes and segmental demyelination were found in sural nerve biopsies.