Electrodiagnostic studies were analyzed in patients with neuropathy associated with IgM monoclonal proteins, 7 with anti-myelin associated glycoprotein reactivity (MAG) and 7 nonreactive to MAG. The findings were distinctly different in the two groups. The electrodiagnostic studies of all the MAG-reactive patients had demyelinating features with slowing of conduction and prolonged distal latencies while only one of the MAG-nonreactive patients had a demyelinating pattern. In fact, the MAG-nonreactive patients were a heterogeneous group, both electrodiagnostically and clinically. This study supports the concept that MAG-reactive polyneuropathy is demyelinating in type, and is caused by the activity of the IgM M-protein directed at an antigen in the myelin sheath.