The electrodiagnostic findings in 51 patients with monoclonal serum proteins of different etiologies and peripheral neuropathies are analyzed and the findings in an additional 10 patients with multiple myeloma are discussed. The nine patients with monoclonal gammopathy of undetermined significance had electrodiagnostic patterns consistent with either predominant axonal degeneration or demyelination. Almost all 15 patients with osteosclerotic myeloma had evidence of a demyelinating neuropathy. The 27 patients with primary systemic amyloidosis almost always displayed evidence of an axonal neuropathy with superimposed carpal tunnel syndrome in some cases. The 10 patients with multiple myeloma had heterogeneous findings. Electrodiagnostic studies can aid in classifying these patients and suggest the likelihood of a specific etiology.