Patients with the pharyngeal-cervical-brachial variant (PCB) of Guillain-Barré syndrome (GBS) have anti-GT1a IgG with or without GQ1b reactivity, whereas those with Fisher syndrome (FS) or Bickerstaff's brainstem encephalitis (BBE) have anti-GQ1b IgG antibodies which cross-react with GT1a. The nosological relationship between these conditions has yet to be established. To investigate the relationships between each manifestation and between clinical features and the coexistence of anti-GQ1b IgG, we reviewed neurological signs present during illnesses of 140 patients who had anti-GT1a IgG. Based on our criteria, FS was diagnosed for 64 (46%) patients, GBS for 22 (16%), BBE for 14 (10%), and PCB for 6 (4%). Overlapping conditions were diagnosed for some patients: FS and GBS (5%), PCB and FS (5%), BBE and GBS (4%), and PCB and BBE (1%). Patients who initially had bulbar palsy developed not only PCB but FS or BBE. The population of anti-GT1a-positive patients frequently had ophthalmoplegia, ataxia, and areflexia, whereas the subpopulation who had anti-GT1a IgG without GQ1b reactivity frequently had preceding diarrhea as well as oropharyngeal, neck, and limb weakness. Patients with anti-GT1a IgG presented a variety of clinical conditions, indicative of a continuous clinical spectrum. A major part of this clinical variation was due to the coexistence of anti-GQ1b IgG. The presence of a common autoantibody (anti-GT1a IgG) and overlapping illnesses suggests that PCB is closely related not only to GBS but to FS and BBE as well.