Antibodies to myelin-associated glycoprotein (MAG) have been demonstrated in the serum samples from about half the patients with polyneuropathy associated with serum IgM monoclonal component. We examined cerebrospinal fluid (CSF) and serum samples from 13 patients with this disease by enzyme-linked immunosorbent assay for anti-MAG IgM antibodies. We detected these antibodies in both CSF and serum samples in 10 of the patients; in three of them the antibodies were at higher levels in the CSF. The remaining three patients had anti-MAG IgM antibodies in the CSF only. Intrathecal production of anti-MAG IgM antibodies is thus common in polyneuropathy associated with IgM monoclonal component. In three patients, examined on two occasions from 1 to 7 years, high anti-MAG IgM antibody levels persisted in CSF and serum samples. Among 165 patients with other neurologic diseases, including 60 with multiple sclerosis and 60 control subjects with tension headache, anti-MAG IgM antibodies were detected in the CSF from three patients (two with multiple sclerosis, one with aseptic meningitis), and in the serum sample of one patient with multiple sclerosis. Whether the frequent occurrence of anti-MAG IgM antibodies in CSF and their intrathecal synthesis has pathogenetic relevance for the development of polyneuropathy associated with IgM monoclonal component is unsure.