Thirteen patients with polyneuropathy associated with plasma cell dyscrasia had serial electrophysiological studies. Five patients with monoclonal IgG had motor and/or sensory symptoms of which 4 correlated with slow motor and sensory nerve conduction. The 4 patients with monoclonal IgM reactive with myelin-associated glycoprotein (MAG), had predominantly motor symptoms, demyelination in the nerve biopsy and slow motor and sensory nerve conduction. Four patients with monoclonal IgM without anti-MAG activity had mainly sensory symptoms, axonal neuropathy on nerve pathology and slow or absent sensory nerve conduction. After treatment with plasmapheresis and chemotherapy 9 patients improved clinically and 4 were unchanged. Criteria for electrophysiologic improvement were presence of sensory or motor responses that were absent before treatment, conduction velocity increased by more than 10 m/s and increase of amplitude by more than 100%. Electrophysiological studies showed improvement in 7, were unchanged in 4, and worse in 2. Sensory velocities in ulnar and sural nerves were significantly improved following treatment (P less than 0.002) and the same trend was noted for the sensory velocity in the median nerve (P less than 0.19). We conclude that nerve conduction studies in combination with clinical examinations are useful in documenting the effects of treatment in these neuropathies.