The presence of a monoclonal protein in the serum of a patient with peripheral neuropathy raises the suspicion of systemic amyloidosis, POEMS syndrome, macroglobulinemia, multiple myeloma, or lymphoma. If these conditions are excluded, the patient is classified as having a monoclonal gammopathy of undetermined significance (MGUS) with an associated neuropathy. Approximately one half of patients with peripheral neuropathy and IgM monoclonal gammopathy have IgM antibodies that bind to a myelin-associated glycoprotein (MAG). In addition to MAG, other antigens such as glycolipids or gangliosides represent other target antigens. Monoclonal gammopathies and motor neuron diseases have also been reported. Sensorimotor peripheral neuropathy occurs in about 15% of patients with primary amyloidosis but is uncommon in multiple myeloma.