Clinical, electrophysiological and pathological findings in 23 patients with subacute and relapsing idiopathic demyelinating polyneuropathies are described. In 17 patients with relapsing polyneuropathy, the neurological illness was unaccompanied by any systemic disturbances. The term preferred for the neuropathy in this group of patients is chronic relapsing polyneuritis. The findings in this group suggest that the common form of this syndrome is due to a single disease entity. Chronic relapsing polyneuritis differs from acute idiopathic polyneuritis chiefly in regard to the rate of evolution and the severity of the initial episode of polyneuropathy. If these two polyneuropathies have the same pathogenesis, the factor which determines whether the disease is acute and self-limiting or chronically relapsing is often present at the time of onset of the disease. The relationship of chronic relapsing polyneuritis to relapsing hypertrophic polyneuropathy and progressive hypertrophic polyneuropathy is also discussed and it is concluded that these diseases may constitute a spectrum of pathogenetically related disorders. In chronic relapsing polyneuritis, as in other demyelinating polyneuropathies, a marked segmental reduction in axon diameter accompanies demyelination. This corresponds to a more than 50% reduction in the volume of the affected region of the axon and it is associated with increased packing of axoplasmic organelles and wrinkling of the axolemma. It is suggested that in the normal myelinated nerve fibre, the Schwann cell and myelin sheath maintain fluid locally within the axon.