Observations have been made on a consecutive series of 62 patients with peripheral neuropathy associated with benign monoclonal paraproteinaemia. The paraprotein class was IgM in 46 cases, IgG in 11 and IgA in 5. Although showing variations between patients, the clinical picture was similar for those with either IgM or IgG paraproteins, usually consisting of a late-onset, slowly progressive, distal sensorimotor demyelinating polyneuropathy, often with tremor and ataxia as prominent features. Tremor was slightly more common in patients with IgM paraproteins, in whom there was a male preponderance. The patients with both paraprotein classes were indistinguishable clinically and electrophysiologically from chronic idiopathic demyelinating polyneuropathy. In the 5 patients with an IgA paraprotein, there was a distal sensorimotor neuropathy in 4 which was demyelinating in 1. In 1 there was proximal demyelinating motor neuropathy. Immunoglobulin deposition on myelin was observed only in the patients with IgM paraproteinaemia, more commonly with a kappa light chain. No deposition of immunoglobulin in the endoneurium was seen. IgM deposits on the perineurium are a feature of normal nerve and were present in all cases. Widely spaced myelin was confined to cases with IgM paraproteins in which immunoglobulin deposition was detected on myelin. The response to treatment could not be assessed systematically but, in general, the patients with IgG and IgA paraproteins responded more satisfactorily (to corticosteroids, cytotoxic drugs, or plasma exchange) than did those with an IgM paraprotein.