In this study, we investigated the affinity of anti-GM1 IgG antibodies as well as their IgG subclass distribution in a series of 38 patients with Guillain-Barré syndrome. In 7 sera elevated titres of IgG anti-GM1 antibodies could be detected. With respect to affinity there were two distinct groups of anti-GM1 antibodies: one group was of high affinity and did not cross-react with other glycolipids; the other group was of low affinity and cross-reacted with asialo-GM1. IgG1 was the predominant and almost exclusive subclass of high affinity anti-GM1 antibodies. Axonal degeneration occurred significantly more frequently in patients with high affinity anti-GM1 antibodies than in patients without anti-GM1 antibodies or in patients with low affinity anti-GM1 antibodies. The presence of anti-Campylobacter jejuni antibodies was not associated with a specific electrophysiological pattern. The prognosis was not dependent on the detection of any of the antibodies, whereas axonal loss and ventilation were associated with a poor prognosis.