We review clinical, neurophysiological, immunological, and experimental data concerning multifocal motor neuropathy (MMN), a newly recognized disorder that mimics MND. It is separated from MND by the presence of multifocal conduction block (CB) demonstrated electrophysiologically, and in some instances by the association of high titers of GM1 antibodies. The possible immunopathogenetic effect of GM1 antibodies is discussed. However, 70% of patients with MMNCB do not have elevated titers of GM1 antibodies, but may respond nevertheless to immunosuppressive treatment. Thus, so far unrecognized antibodies may react against some other epitopes in the paranodal region than those attacked by GM1 antibodies to cause CB.