Guillain-Barré syndrome (GBS) is a postinfectious autoimmune polyradiculoneuropathy. The most frequent antecedent pathogen is Campylobacter jejuni, followed by cytomegalovirus. However, more than 40% of GBS cases currently cannot be attributed to triggering events. This might be due to the shortcomings of the serological assays used for diagnosing infections, in particular for C. jejuni. In our study investigating 36 patients with acute GBS, standard serological methods identified the triggering viral or bacterial etiology in only 25% of cases. However, using a highly specific enzyme-linked immunosorbent assay based on two recombinant outer antigens encoded by C. jejuni genes Cj0017 (P39) and Cj0113 (P18), we found serological evidence of a preceding C. jejuni infection in 80.6% of the patients but in only 3.5% of the controls. We conclude that the role of C. jejuni in triggering GBS has been greatly underestimated.