Guillain-Barré syndrome (GBS) is an acute autoimmune neuropathy, often preceded by an infection. Serum anti-ganglioside antibodies are frequently elevated in titer. Those antibodies are useful for diagnosis. Some of them also may be directly involved in the pathogenetic mechanisms by binding to the regions where the respective target ganglioside is specifically localized. We have recently found the presence of the antibody that specifically recognizes a new conformational epitope formed by two gangliosides (ganglioside complex) in the acute-phase sera of some GBS patients. In particular, the antibodies against GD1a/GD1b and/or GD1b/GT1b complexes are associated with severe GBS requiring artificial ventilation. Some patients with Miller Fisher syndrome also have antibodies against ganglioside complexes including GQ1b; such as GQ1b/GM1 and GQ1b/GD1a. Gangliosides along with other components as cholesterol are known to form lipid rafts, in which the carbohydrate portions of two different gangliosides may form a new conformational epitope. Within the rafts, gangliosides are considered to interact with important receptors or signal transducers. The antibodies against ganglioside complexes may therefore directly cause nerve conduction failure and severe disability in GBS. More study is needed to elucidate the roles of the antibodies against ganglioside complexes.