Anti-GQ1b IgG frequently is present in sera of patients with Miller Fisher syndrome (MFS), Guillain-Barré syndrome (GBS) with ophthalmoplegia, Bickerstaff's brainstem encephalitis (BBE), and acute ophthalmoparesis (AO) in the acute phase. Why various clinical signs develop under these conditions, however, has yet to be clarified. We investigated the fine specificity of anti-GQ1b IgG and its clinical correlation in sera from 82 patients: 56 with MFS, 11 with GBS, 13 with BBE, and 2 with AO. Anti-GQ1b IgG antibodies were absorbed by GT1a in 80 (98%) of the 82 sera, by GD1b in 11 (13%), and by the other b-series gangliosides GD3, GD2, or GT1b in 24 (29%). The most frequent pattern of fine specificity was the cross-reaction with GT1a alone, seen in 56 (68%) samples. Of the 11 patients with anti-GQ1b IgG, cross-reacting with GD1b, 6 (55%) had impaired deep sense, and the association was significant (p=0.02). This is the first study to show that the fine specificity of anti-GQ1b IgG is heterogeneous and that the difference is correlated with the presence of a particular clinical sign.