A randomised controlled trial of exchange versus no exchange was conducted to find out whether plasma exchange would be useful in acute inflammatory polyradiculoneuropathy. It was calculated that 15 patients would be required in each group to demonstrate a worthwhile improvement in functional ability 1 month after completion of treatment. Treatment comprised five exchanges in 10 days (55 ml plasma/kg body weight/exchange). Both groups received normal supportive care and were followed up periodically for a year. Overall the treated group showed a slight but not significant benefit (p greater than 0.05); at two weeks' follow-up of patients admitted to the trial within 14 days of onset of neuropathic symptoms, p = 0.07. These results do not provide grounds for recommending plasma exchange for the treatment of severe AIP.