Multifocal motor neuropathy (MMN) was first distinguished from other motor neuropathies in 1986. It is characterised by slowly progressive predominantly distal and asymmetric limb weakness and wasting that predominates in the arms, with muscle cramps and fasciculations, within an anatomical distribution of individual motor nerves. Sensory involvement is minimal or absent. The electrodiagnostic hallmark is focal motor conduction block (CB), persisting for years at atypical sites. The most typical laboratory finding is increased serum IgM autoantibody titers to the ganglioside GM1. High-dose intravenous immunoglobulin (IVIg) is currently the gold-standard treatment for MMN. To document short-term and long-term responses to IVIg, we conducted a retrospective study of 40 patients with MMN defined using ENMC Workshop criteria and treated with periodic IVIg infusions (Tégéline) between 1995 and 2003. For the short-term analysis we compared the 22 patients who had never previously received IVIg with the 18 IVIg-experienced patients. For the long-term evaluation (> 6 months), the patients were classified into four groups according to their dependence on periodic IVIg. The MRC score improved significantly in 14 (70% ; 95% CI 0.46 to 0.88) of the 20 assessable treatment-naive patients (data were missing for two patients). This rate was significantly higher than at six months in a historical group of placebo-treated patients (20%; p < 0.0001). No criteria predictive of the response were identified. At the end of follow-up (mean 2.2 +/- 2.0 years) only 8 patients (22%) in the cohort remained in remission after a good initial response to IVIg, while 25 patients (68%) were dependent on periodic IVIg infusions. This study confirms the good short-term response of MMN to IVIg but indicates that the longer-term results are variable. New therapeutic strategies are required to increase the short-term and long-term efficacy of IVIg, and to reduce reliance on this treatment.