We reviewed our data from patients with the clinical diagnosis of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). Seventy patients had no demonstrable underlying disease to account for their polyneuropathy and were classified as idiopathic CIDP (CIDP-I). We detected a monoclonal gammopathy of uncertain significance (MGUS) in 30 patients who were classified as CIDP-MGUS; 17 had an IgG gammopathy, 12 an IgM gammopathy, and one an IgA gammopathy. Compared with CIDP-I patients, CIDP-MGUS patients were older and slightly more likely to be males. When compared with patients with an MGUS but without polyneuropathy reported in the literature, CIDP-MGUS patients had similar distributions of age, sex, and immunoglobulin class. There were no significant differences in motor and sensory nerve conduction measures between CIDP-I and CIDP-MGUS patients, nor between CIDP-MGUS patients with IgM and those with IgG or IgA gammopathy. Strict electrodiagnostic criteria for primary demyelination were fulfilled by 54% of CIDP-I patients and 40% of CIDP-MGUS patients, but these were not significantly different. Our study suggests that (1) the demographic features and immunoglobulin class distribution of CIDP-MGUS patients largely reflect those of patients with an MGUS, but without polyneuropathy, (2) CIDP-MGUS patients as a group cannot be distinguished from CIDP-I patients on the basis of nerve conduction studies, and (3) IgM CIDP-MGUS patients cannot be distinguished from those with other immunoglobulin classes.