We analyzed the characteristics of 29 Guillain-Barré syndrome (GBS) patients with IgM anti-GalNAc-GD1a antibodies. Fourteen of them had had an antecedent cytomegalovirus infection (CMV group) and 12 gastrointestinal infection (G-I group). Most of the G-I group patients (nine of 12) were subsequent to Campylobacter jejuni infection. Electrophysiological results in both groups patients predominantly indicated demyelinating neuropathy. The CMV group patients were characterized by slow progression and frequent facial and sensory deficits. IgM antibodies in their sera recognized an epitope shared by GalNAc-GD1a and GM2. The G-I group patients frequently showed motor type of GBS with cranial nerves spared. IgM antibodies specific to GalNAc-GD1a were present in their sera.