The electrodiagnostic studies of 13 consecutive patients with multifocal sensory and motor neuropathy of unknown etiology were reviewed to determine whether they exhibit features of demyelination or axonal degeneration. The type and frequency of demyelinating features, fulfillment of electrodiagnostic criteria for chronic inflammatory demyelinating polyneuropathy (CIDP), and response to immunotherapy were noted. Of 13 patients, 11 had at least one electrodiagnostic feature of demyelination at presentation and 2 had none. Seventeen percent to 77% of the patients fulfilled at least one of the published electrodiagnostic CIDP criteria, depending on the criteria used, but the number of demyelinating features per patient was less than reported for unselected patients with CIDP. Patients with multifocal sensory and motor neuropathy had a similar percentage of nerves with partial conduction block or F-wave prolongation as reported for unselected CIDP, but a smaller percentage of nerves exhibiting prolonged distal compound muscle action potential duration, distal latency prolongation or slowed conduction velocities. All treated patients, including 2 who did not meet any CIDP criteria, had at least a moderate response to immunotherapy. The results indicate that a large majority of, but not all, patients with idiopathic multifocal sensory and motor neuropathies exhibit electrodiagnostic features of demyelination, although fewer than seen in classic CIDP.