The Guillain-Barré syndrome is the most common cause of acute flaccid paralysis. The current mortality is 5%, while severe motor sequelae persist after one year in 10%. Multidisciplinary teams, trained in all specific treatments, are required to treat these patients. Oral and intravenous steroids have proved ineffective. Two large randomized clinical trials comparing plasma exchange (PE) with no treatment have shown a short-term and a 1-year benefit. The appropriate number of exchanges and the indications are now more precisely known. In the mild form (walking possible) patients should receive two PEs; a further two exchanges should be done in the event of deterioration or in advanced forms (loss of walking ability, mechanical ventilation). More exchanges are not beneficial. Recently two new randomized trials have produced evidence that intravenous immunoglobulin (IVIg) (0.4 g/kg/day for 5 days) was as effective as 5 PEs in advanced forms. The combination of PE with IVIg did not confer a significant advantage but increased the costs and risks. In advanced forms the choice between PE and IVIg depends on the contraindications for each treatment.