In vitro electrophysiologic and light microscopic studies were carried out on the sural nerve in six patients with non-leprous neuropathy with plantar ulceration and in six patients with various types of leprous neuropathy. In the non-leprous group (with congenital and acquired neuropathy) the abnormalities in the compound action potentials of the myelinated (large and small) fibers were usually more striking than those in the unmyelinated fibers potentials. In the leprous neuropathies, on the other hand, the three major fiber groups tended to be involved indiscriminately, the unmyelinated fiber potential being as liable to abnormality as the myelinated fiber potentials. Histologically the nerve fiber damage in the congenital neuropathies appeared to be moderate to gross involvement of the myelinated fibers without the prominent demyelination and degeneration/regeneration seen in leprosy and the other acquired neuropathies. Leprous neuropathy showed, besides fiber abnormalities, the simultaneous deleterious effects of inflammation and fibrosis. It is a matter for speculation whether disturbed conduction in the large myelinated touch-pressure mediating fibers contributes significantly to impaired pain perception (mediated by small fibers) and plantar ulceration in leprosy and other acrodystrophic neuropathies.