During 1 year, we used immunocytochemical staining of human cerebellum to screen 1,488 serums for IgG autoantibodies to Hu and Yo antigens. Three serums had none of the classically described patterns of IgG binding but, instead, selectively stained the cerebellar molecular layer. Evaluation of clinical data showed that the patients had either typical Miller Fisher syndrome (MFS) or Guillain-Barre syndrome with ophthalmoplegia. Further analysis by ELISA assay showed that all three serums had high titers of IgG anti-GQ1b autoantibodies. IgG autoantibody staining of human cerebellum, which is used for the diagnosis of paraneoplastic disorders, may have additional specificity for other, presumably autoimmune, syndromes such as MFS. The specificity of the serum IgG autoantibody binding to the cerebellar molecular layer may be related to the ataxia that often occurs in these patients.