The indistinctness of the HMSN type III concept of Dyck (1975) prompted us to evaluate the diagnostic criteria. Based on a literature review and the observations in five of our own cases, restricted criteria are formulated. We conclude that at present the diagnosis of AR HMSN type III can be applied reasonably to the condition of "congenital hypomyelination", which shows a congenital or early childhood onset, extremely slow motor nerve conduction velocities of less than 6-7 m s-1 in upper limbs, and in nerve biopsy only fibres with no or hardly any myelin and "onion bulbs" of basal lamina. Amyelination might be the most severe or earliest expression of congenital hypomyelination. The existence of an inherited type III with mainly classical onion bulbs is uncertain, as only sporadic cases have been described.