Many data point to a pathogenetic role for IgM antibodies to the myelin-associated glycoprotein (MAG) in the neuropathy associated with IgM monoclonal gammopathy, supporting the use of immune therapies in affected patients. Almost 50% of patients have been reported to improve with these therapies, but the effect of treatment on the long-term prognosis of the neuropathy remains unclear. We analysed the outcome of 25 of the 26 patients (mean age at entry 65 years, range 45-85 years) with neuropathy and high anti-MAG IgM, first examined by us between 1984 and 1994. By January 1999, after a mean follow-up of 8.5 years (range 2-13 years) and a mean duration of neuropathy symptoms of 11.8 years (range 3-18, >10 years in 16), 17 patients (68%) (aged 58-84 years, mean 73.4) were alive, while eight (32%) (aged 69-78 years, mean 73.1) had died 3-15 years (mean 10.6) after neuropathy onset; in none of them was death caused by the neuropathy, although in three it was possibly related to the therapy for the neuropathy. By the time of last follow-up or patients' death, 11 patients (44%) were disabled by severe hand tremor, gait ataxia or both. The disability rates at 5, 10 and 15 years from neuropathy onset were 16, 24 and 50%, respectively. Of the 19 patients treated during the follow-up for 0.5-11 years (mean 4 years) with various immune therapies, five reported a consistent and four a slight improvement in the neuropathy (total 47%) after one treatment or more, but in only one patient was improvement persistent throughout, to the end of follow-up. In 10 patients (53%), severe adverse events, possibly related to therapy, occurred during treatment and were considered responsible for the patient's death in three. The neurological impairment did not differ between treated and untreated patients at the end of a similar follow-up. Our findings indicate that (i) the majority of patients with neuropathy and anti-MAG IgM have a favourable prognosis even after several years, and (ii) current immune therapies, though temporarily effective in half of the patients, are associated with considerable side effects which limit their prolonged use and efficacy, suggesting that until more effective or safer therapies become available, they should probably be reserved for patients impaired in their daily life or in a progressive phase of the disease.