Campylobacter curvus and Campylobacter upsaliensis were isolated from stools of patients with Guillain-Barré (GBS) or Fisher's (FS) syndromes. Whether these microorganisms are pathogens of antecedent diarrhea in GBS and FS is not clear, therefore, we made a serological examination. There were no differences in antibody titer to these organisms among the patients with GBS, FS, and the controls. Some patients had elevated antibodies to the bacteria, but most also had serological evidence of C. jejuni infection. Moreover, the patients from whom C. curvus had been isolated did not have antibodies to the bacterium, indicative that they were healthy carriers of C. curvus or that the isolates were the product of contamination. We conclude that neither C. curvus nor C. upsaliensis is the major agent of antecedent diarrhea in GBS and FS.