The diagnostic criteria, natural history, nerve conduction characteristics, pathology, laboratory features, and efficacy of corticosteroid treatment have been evaluated personally in 53 patients with chronic inflammatory polyradiculoneuropathy (CIP) who were followed up for an average of about 7.5 years. These were patients whose monophasic neurologic deficit had not crested by 6 months, patients with recurrences, and patients with a steady or stepwise progression. The typical features of CIP include absence of an associated disease, frequent history of preceding infection or receipt of foreign protein, and tendency to involve cranial, truncal, and proximal as well as distal limb structures and to have diffusely slow conduction velocity of peripheral nerves. The most marked slowing is often very proximal. The pathologic features include serous edema, mononuclear cell infiltrates (especially in perivascular areas, but without evidence of vasculitis), macrophage-induced segmental demyelination, and hypertrophic neuritis. If our patients are representative, complete recovery occurs only infrequently; about 60% of patients are able to be ambulatory and work, 25% become confined to a wheelchair or become bedridden, and approximately 10% die from their disease. Although the bulk of the pathologic changes affect spinal roots and proximal nerves, the brain and spinal cord may be involved also. Degeneration into linear rows of myelin ovoids is the predominant type of myelinated fiber degeneration of the sural nerve at the ankle.