Twenty-five patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) were studied in order to define the clinical, biological, electrophysiological and pathological features of this disease. There were 11 men and 14 women ranging in age from 15 to 82 years. The average follow-up was 43 months. Patients fulfilled the criteria laid down by Dyck et al. (1975), except that progression of weakness was at least 2 months and not 6. Fourteen patients had a progressive course and 11 a relapsing one. Weakness was almost constant (24/25), sensory impairment was present in 22/25 with deep sensation predominantly impaired. Areflexia was observed in all patients. A history of previous infection or other possible precipitating event was given by 7 patients. Cerebrospinal fluid examination showed a raised total protein count in 22 cases. Electrophysiological examination revealed a slowing down of nerve conduction velocities to an extent compatible with a demyelinating process in 23 cases; 2 patients only had prolongation of F-waves. Sural nerve biopsy was less informative: inflammatory process--the most specific finding--was observed in only 3 out of 20 biopsies. Six patients had benign forms or spontaneous remission, the others were treated with corticosteroids or with immunosuppressive drugs. Most patients (64%) recovered very well. Only one died during the time of study. Eight other patients were treated with high-dose intravenous immunoglobulins and 4 of them improved with administrations at regular intervals to maintain the benefits observed. The occurrence of CIDP in association with other conditions is reviewed and we discuss its nosological position among the acquired demyelinating neuropathies.