IgA, IgG, and IgM antibodies against ganglioside GM 1 were investigated in sera of 15 patients with Guillain-Barré syndrome (GBS), as compared with healthy controls. Significantly raised titers were found in 4 patients who showed persisting weakness due to muscle wasting (3 patients) or due to the development of a chronic relapsing form of GBS (1 patient) after a follow up period of 1 year. By contrast, only 1 out of 11 anti-GM 1 seronegative patients showed persisting muscle wasting. In 1 patient with GBS following Campylobacter jejuni enteritis an IgA anti-GM 1 immune response was found to predominate, suggesting that this immune response may be due to crossreactivity between GM 1 and Campylobacter antigens. We conclude that elevated antibody titers against ganglioside GM 1 may be a valuable marker to predict poor motor recovery in GBS.