The authors studied prospectively with electrophysiological means, peripheral nerve maturation of 18 neonates with congenital hypothyroidism compared to 18 controls. Before replacement therapy, a large reduction of sural nerve action potential amplitude and of Hoffmann's reflex was observed. It was accompanied by a moderate reduction in nerve conduction velocity. These results imply a myelinic and even more an axonal pathology. It is related to the anatomical type of hypothyroidism and to serum T4 deficiency. It is transient and disappears after 6 months of hormonal replacement therapy. In conclusion, this study gives new informations on the consequences of congenital hypothyroidism on peripheral nerves.