Anti-fucosyl-GM1 ganglioside antibodies were detected in sera of five persons: four patients with autoimmune neuropathies and more recently, IgG antibodies in one with Graves' disease (Adler et al., Autoimmunity 18, 149-152, 1994) [1]. In the latter case, we were unable to find any relation between the occurrence of antibodies and thyroid disease. Now we report a detailed study on the anti-glycolipid antibodies in this patient. We found that her serum contained not only IgG but also a high level of anti-FucGM1 IgM antibodies, with a titer stable over a period of 5 years of treatment and follow-up. The carbohydrate structure of the epitope recognized by IgG and some of IgM antibodies seems to consist of Fuc-Gal-GalNAc-Gal- or a part of this sequence. Moreover, this patient's serum contained other IgM antibodies active against FucGM1 and also asialo GM1 glycolipids. Our results indicate that anti-FucGM1 ganglioside antibodies of G and M classes occur in serum of this patient with no apparent adverse health effects.