Polyneuropathies associated with IgM monoclonal gammopathy (IgM-MG) are the most frequent type of peripheral neuropathy (PN) associated with monoclonal gammopathy. The pathogenic relevance of IgM-MG in PN is supported by several arguments, the more significants are pathological data on neuromuscular biopsies and the demonstration of IgM antibody activity to peripheral nerve antigens, mainly myelin-associated-glycoprotein (MAG) and sulfated glucuronyl paragloboside (SGPG). In addition recently reported series show that there is a significant correlation between clinical/electrophysiological data of PN, and IgM-MG antibody activity, in more than 75% of cases. Those features are in favour of an immune-mediated disease.