Some patients developed Guillain-Barré syndrome (GBS) after the administration of bovine gangliosides. Patients with GBS subsequent to Campylobacter jejuni enteritis frequently have IgG antibody to GM1 ganglioside. Miller Fisher syndrome (MFS), a variant of GBS, is associated with IgG antibody to GQ1b ganglioside. We showed the existence of molecular mimicry between GM1 and lipopolysaccharide of C. jejuni isolated from patients with GBS, and that between GQ1b and C. jejuni lipopolysaccharides from patients with MFS. The molecular mimicry between infectious agents and gangliosides may function in the production of anti-ganglioside antibodies. This sugar mimicry is one possible cause for GBS and MFS, and unidentified host factor may contribute to the development of these syndromes.