Guillain-Barré syndrome (GBS) is defined clinically as a peripheral neuropathy causing limb weakness that progresses for up to 4 weeks before reaching a plateau. The symptoms may be caused by inflammatory demyelination, axonal degeneration, or both. GBS occurs throughout the world, with a median incidence of 1.3 cases/100,000 population (range, 0.4-4.0). Males are more commonly affected than females, and there are peaks in young adults and the elderly. There is no clear seasonal association in Western countries, although this may be because the most frequent antecedent events, respiratory and enteric infections, have opposite seasonality. The most frequently identified cause of GBS is Campylobacter jejuni infection, which has been identified in up to 41% of patients and is associated with more severe disease and prolonged disability. Summer epidemics of GBS occur among children and young adults in Northern China and are particularly likely to be associated with C. jejuni infection.