The diagnosis of multifocal motor neuropathy (MMN) is often missed because MMN disguises itself as a motor neuron disease and is considered relatively rare. Detailed epidemiological studies of MMN have not been undertaken. We therefore conducted a nationwide survey of MMN in comparison with amyotrophic lateral sclerosis (ALS). This retrospective study examined 47 patients with MMN and 1,051 patients with ALS from major neuromuscular centers in Japan from 2005 to 2009. MMN had a younger age of onset and was more common in males than ALS. The ratio of MMN to ALS patients (0-0.10) varied among the centers, but mostly converged to 0.05. The prevalence was estimated to be 0.3 cases for MMN and 6.63 cases for ALS per 100,000 persons. Twenty-five MMN patients (54.2%) showed conduction block. Thirty-four (75%) of 45 MMN patients received intravenous immunoglobulin exhibited a favorable outcome. It is expected that more sensitive indicators of conduction block or focal demyelinating lesions than currently available MMN diagnostic criteria would further increase the ratio of MMN to ALS patients and the total number of MMN patients who can benefit from treatment. MMN is by no means a rare disorder but should be accurately diagnosed in all neuromuscular centers.