Three patients who had diarrhea prior to the development of Miller Fisher syndrome are presented. Campylobacter jejuni was isolated from stool specimens from all patients. High titers of anti-GQ1b IgG antibodies were demonstrated in the serum of these patients by enzyme-linked immunosorbent assay and thin-layer chromatography overlay. In enzyme-linked immunosorbent assay inhibition studies the anti-GQ1b IgG antibodies bound specifically to whole bacteria of the Miller Fisher syndrome-associated C. jejuni strains. The presence of anti-GQ1b IgG binding epitopes on the surface of the C. jejuni from the patients was not exclusively associated with a specific Penner serotype. It is suggested that anti-GQ1b antibodies are formed during the initial infection that elicits Miller Fisher syndrome. The cross-reactivity of anti-GQ1b IgG antibodies with surface epitopes on Miller Fisher syndrome-associated C. jejuni strains supports the hypothesis of molecular mimicry between bacteria and neural tissue.