Complement activation is involved in the pathogenetic mechanism of Guillain-Barré syndrome (GBS). To date, the effectiveness of complement inhibitors for GBS has been shown by in vitro and in vivo studies. A recent Japanese randomized controlled trial with eculizumab, a monoclonal antibody against the complement C5, indicated that eculizumab might improve the outcomes of GBS patients at six months from onset. In future, the prognosis of severe GBS cases may possibly be improved by a novel therapy targeting the complement.