Six children suffering from a congenital motor and sensory neuropathy (CMSN) are described. Severe muscle hypotonia, areflexia and a delay of motor development are detectable in all of them. Sural nerve biopsies exhibited an almost complete absence of myelinated fibres and a correspondingly slow nerve conduction velocity (NCV) of less than 10 m/s was detectable in four patients. A few segments with hypermyelination adjacent to gross hypomyelination were seen in the fifth patient, and the NCV was 15 m/s. The sural nerve of the sixth patient showed a loss of thick myelinated nerve fibres, and his NCV was 25 m/s. These results demonstrate the histological heterogeneity of CMSN which was already detected by the NCV. The relation of our findings to the classification of HMSN by Dyck and Lambert (1968) is discussed.