Clinical assessment, CSF changes, electrophysiologic findings, and nerve biopsy investigations are reported for ten cases of chronic idiopathic polyneuritis. Seven cases developed a progressive, three a relapse course. Motor deficit syndromes predominated, often with proximal weakness. Sensory signs were less frequent and less intense, although exceptions were noted. Elevated CSF total protein and a disproportional increase in IgG developed, which increased with duration of the disease. Nerve conduction velocity was markedly reduced. Nerve biopsies showed cell-mediated demyelination, simultaneous de- and remyelination, considerable alteration of Schwann cells, and long-persisting glycolipid-rich myelin degradation products. Particular features of chronic polyneuritis are delineated. The demyelinating process seems to be "self-perpetuating" and not monophasic as the acute forms. Schwann cells or their surface antigens seem to be targets of the immune response. Chronic polyneuritis responds to treatment with corticosteroids. Long-term treatment in combination with immunosuppressants and plasmapheresis is often required.