We performed clinical and electrophysiological studies in 42 children with hereditary motor and sensory neuropathy type I and II (HMSN I and HMSN II) and in 103 members of their families. In 24 families with HMSN I the conduction velocity and the latency were markedly changed in the nerves innervating the distal muscles (median, peroneal nerves), as well as proximal muscles (facial, axillary, and musculocutaneous nerves). The changes were uniform in all motor and sensory nerves studied in the particular patient. No nerve conduction worsening with age has been found in cross-sectional analysis. In patients with HMSN I the conduction velocity was impaired even when the clinical abnormalities were minimal. The degree of the conduction velocity slowing was uniform within majority of the families. Homogeneity of conduction velocity slowing in individuals with HMSN I regardless of clinical expression suggests a primary myelin defect as an underlying cause. In patients from 18 families with HMSN II slight changes in conduction velocity were found only in the nerves innervating the distal muscles, the latency of axillary and facial nerves was within normal range. We recommend examining conduction time in facial and axillary nerves as a useful procedure for differentiation between HMSN I and II, especially in families with borderline conduction values in the long nerves.