Polyneuropathies associated with IgM monoclonal gammopathies comprise a distinct entity. In spite of the apparent pathogenicity of the IgM antibodies and the specific immunoreactivity to myelin antigens, the disease has been difficult to treat. This review describes the clinical phenotype, addresses recent data on immunoreactivity of IgM to various nerve antigens, and discusses the latest progress on treatment.Most of these patients present with paresthesias and sensory ataxia followed by a varying degree of sensorimotor deficits. In more than 75% of the patients, the monoclonal IgM recognizes myelin-associated glycoprotein (MAG) and sulfoglucuronyl glycosphingolipid (SGPG), best detected by ELISA, or other peripheral nerve glycolipids. Recent experiments have demonstrated that animals immunized with SGPG develop sensory ataxia, suggesting a pathogenic role for this antigen. Although cladribine, cyclophosphamide with prednisone, and intravenous immunoglobulin have offered transient benefits to some patients, most have remained treatment-resistant. Open label studies and a recent randomized controlled trial indicate that rituximab is emerging as the best agent available, providing long-term benefits to almost half of these patients. Rituximab appears to work by suppressing the IgM as well as the anti-MAG antibodies and by inducing immunoregulatory T cells. Patients with more sensory deficits and higher anti-MAG antibodies are more likely to respond but may require re-treatment after several months.These encouraging results need confirmation with a larger trial. Data on long-term efficacy and immune markers associated with response to therapy or need for re-treatment are still needed.