Ninety-two patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) have been studied in order to define better the clinical features, course and prognosis of the condition and to identify possible aetiological factors. Sural nerve biopsy was performed on 87 subjects. Electrophysiological studies were undertaken on all patients and demonstrated marked slowing of motor conduction and impairment of sensory conduction. The onset was usually gradual but there was a rapid rate of onset in 15 (16%) patients. Males were more commonly affected than females. Weakness and paraesthesiae were the most common symptoms but pain was frequently a feature. Age of onset ranged from 2 to 72 years. Sixty patients (65%) had a relapsing course and 32 patients (35%) a progressive or monophasic course; there was a significantly earlier age of onset in patients with relapsing disease. Thirty-two patients (35%) gave a history of preceding infection or some other possible antecedent precipitating event and there was a significantly higher titre for cytomegalovirus antibodies in the serum of patients with CIDP than in controls. The patients were followed up for an average time of approximately ten years. Most patients (73%) had made a good recovery and were independent, but 7 patients had either died or were completely immobilized as a result of their disease. The value of treatment with corticosteroid therapy, immunosuppressive agents and plasma exchange is discussed.