The clinical and histopathological features in sural nerve biopsies from 10 cases of dominantly inherited hypertrophic Charcot-Marie-Tooth disease (hereditary motor and sensory neuropathy (HMSN), type I) presenting in childhood are contrasted with those of 6 cases of Dejerine-Sottas disease (HMSN type III). There was a significantly greater incidence of ataxia, areflexia and clinical nerve enlargement in HMSN type III. In HMSN type III, there was also a significantly lower density of myelinated fibres 8 microns or more in diameter, a greater frequency of onion bulbs, more lamellae per onion bulb and, a higher ratio of mean axon diameter to fibre diameter. The functional severity of the HMSN type III cases was not markedly worse than those with HMSN type I. Using these parameters, it was possible to attempt classification of sporadic cases of hereditary motor and sensory neuropathy. Cerebrospinal fluid protein levels were unreliable in distinguishing the two types of neuropathy.