Human and animal studies on antibody-mediated neuropathy implicate complement in pathogenesis. In animal models complement inhibition is therapeutically beneficial. The monoclonal antibody, eculizumab (Soliris™, Alexion Pharmaceuticals, Cheshire, CT), prevents cleavage of C5 and thus inhibits terminal complement activation. In an open label study, 13 multifocal motor neuropathy patients received eculizumab for 14 weeks, 10 of whom were concomitantly receiving intravenous immunoglobulin. The primary outcome was safety of eculizumab, and the secondary outcomes included change in intravenous immunoglobulin (IVIg) dosing frequency, performance, and electrophysiological parameters. Adverse events were minor during the study. Nine of 10 patients on IVIg maintenance continued to require IVIg. IVIg dosing interval was not different between the run-in and the treatment period. There were improvements in patient-rated subjective scores and selected clinical and electrophysiological measurements. Overall, a small treatment effect occurred in some patients that appeared supplementary to and independent of the IVIg treatment effect, and occurred more frequently in patients with higher baseline motor function.