IgA has an important function in the gastrointestinal immune system. We investigated IgA anti-ganglioside antibodies in Guillain-Barré syndrome (GBS) and Fisher's syndrome (FS) subsequent to Campylobacter jejuni enteritis. In previous studies, serological diagnosis of C. jejuni infection was based on the detection of IgG, IgA, and IgM anti-C. jejuni antibodies. Our study, however, showed that the detection of IgG anti-C. jejuni antibody alone was sufficient for the serological diagnosis of antecedent C. jejuni enteritis in GBS and FS, when the cut-off level was defined for results of sera from C. jejuni-isolated patients. Serological evidence of C. jejuni infection was found in 62 (31%) of 201 GBS patients and 12 (18%) of 65 FS patients. IgA anti-GMI antibody was detected in sera from 33 (16%) of the GBS patients, 1 (2%) of the FS patients, and none of the 46 normal control subjects. IgA anti-GM1 antibody titers were significantly higher in the GBS patients with positive C. jejuni serology than in those with negative serology (P < 0.0001) or the FS patients with positive C. jejuni serology (P = 0.007). IgA anti-GQ1b antibody was detected in sera from 18 (28%) of the FS patients, 9 (4%) of the GBS patients, and none of the normal control subjects. FS patients with positive C. jejuni serology had significantly higher titers of IgA anti-GQ1b antibody than those with negative serology (P = 0.01) or the GBS patients with positive C. jejuni serology (P < 0.0001). We conclude that anti-GM1 and anti-GQ1b IgA antibodies are closely associated with antecedent C. jejuni enteritis in GBS and FS, respectively.