Anti-myelin-associated glycoprotein (MAG) neuropathy, which occurs secondary to immunoglobulin (Ig)M paraproteinemia such as monoclonal gammopathy of undetermined significance, is characterized by slow progression, sensory or sensorimotor disturbances, and ataxia. The estimated prevalence of this neuropathy in Japan is 0.28 per 100,000 population with male preponderance. This neuropathy is diagnosed based on the detection of M protein and anti-MAG antibodies in patients' serum. Nerve conduction studies show prolonged distal latency, and histopathological evaluation of sural nerve biopsies shows widely spaced myelin on electron microscopy. Usually, immunotherapy, including administration of intravenous Ig and corticosteroids, is ineffective, and rituximab is beneficial in approximately 50% of patients. Novel therapies, such as administration of Bruton's tyrosine kinase inhibitors are expected to benefit patients with the MYD88L265P mutation.