We performed a long-term follow-up study of 11 patients with multifocal motor neuropathy (MMN) who received maintenance treatment with intravenous immunoglobulins (IVIg). Patients were treated initially with one full course of IVIg (0.4 g/kg for 5 days) followed by one IVIg infusion (0.4 g/kg) every week. During follow-up, the frequency and dosage of IVIg infusions were determined for each patient and ranged from one infusion every 1 to 7 weeks and an average dose of 7 to 48 g per week. During the 4- to 8-year follow-up period, muscle strength was assessed by measuring the MRC (Medical Research Council) sumscore of 20 muscle groups and by performing hand-held dynamometry on a selection of weak muscle groups. Systematic electrophysiological studies were performed before treatment and each year during IVIg maintenance treatment. Disability was assessed with the upper limb and lower limb subscales of the Guy's Neurological Disability Scale before treatment, after the first full course of IVIg and at the last follow-up examination. Muscle strength improved significantly within 3 weeks of the start of IVIg treatment and was still significantly better at the last follow-up examination than before treatment, even though it decreased slightly and significantly during the follow-up period. Upper limb disability was significantly better after the first full course of IVIg than before treatment. Conduction block disappeared in six nerve segments but new conduction block appeared in eight nerve segments during the follow-up period. Changes consistent with improvement (remyelination or reinnervation) occurred in 13 nerves during follow-up and changes consistent with worsening (demyelination or axon loss) occurred in 14 nerves. Electrophysiological changes consistent with improvement were significantly associated with the presence of conduction block before IVIg treatment. In conclusion, IVIg maintenance treatment has a beneficial long-term effect on muscle strength and upper limb disability but may not prevent a slight decrease in muscle strength. The electrophysiological findings imply that IVIg treatment favourably influences the mechanisms of remyelination or reinnervation but that axon loss cannot be prevented.