The subacute demyelinating polyneuropathy known as Guillain-Barré syndrome improves more rapidly with plasma exchange than with supportive care alone. We conducted a multicenter trial to determine whether intravenous immune globulin is as effective as the more complicated treatment with plasma exchange. To enter the study, patients had to have had Guillain-Barré syndrome for less than two weeks and had to be unable to walk independently. They were randomly assigned to receive either five plasma exchanges (each of 200 to 250 ml per kilogram of body weight) or five doses of a preparation of intravenous immune globulin (0.4 g per kilogram per day). The predefined outcome measure was improvement at four weeks by at least one grade on a seven-point scale of motor function. After 150 patients had been treated, strength had improved by one grade or more in 34 percent of those treated with plasma exchange, as compared with 53 percent of those treated with immune globulin (difference, 19 percent; 95 percent confidence interval, 3 percent to 34 percent; P = 0.024). The median time to improvement by one grade was 41 days with plasma exchange and 27 days with immune globulin therapy (P = 0.05). The immune globulin group had significantly fewer complications and less need for artificial ventilation. In the acute Guillain-Barré syndrome, treatment with intravenous immune globulin is at least as effective as plasma exchange and may be superior.