Multifocal motor neuropathy (MMN) is an immune-mediated disorder characterized by slowly progressive asymmetrical limb weakness. Treatment with immunoglobulins (IVIg) leads to improvement of muscle strength. Anecdotal evidence suggests that immunosuppressive drugs as adjunctive therapy may be beneficial. Mycophenolate mofetil (MMF) is a potent and safe immunosuppressant. Safety and efficacy of MMF as adjunctive therapy for MMN patients receiving IVIg maintenance treatment were evaluated in a randomized controlled trial. MMN patients responding to IVIg treatment were eligible for randomization. Muscle strength and functional status were assessed at monthly intervals for 1 year. Three months after the start of MMF or placebo treatment, IVIg doses were reduced stepwise, until a deterioration of functioning or decline in muscle strength could be observed. An IVIg dose reduction of 50% during adjunctive treatment was defined as a primary endpoint. Secondary outcome measures were improvement in muscle strength and functional status after 3 months and reduction of anti GM1-IgM titres after 12 months of MMF treatment. Twenty-eight patients were randomized. One patient allocated to MMF reached the primary endpoint of 50% IVIg dose reduction. After 12 months IVIg reduction did not differ significantly between the two treatment groups. Patients did not experience drug toxicity and none of the patients showed significant disease progression after 12 months. Muscle strength and functional scores after 3 months and anti GM1-IgM titres after 12 months did not change. Adjunctive treatment of MMN patients with MMF at a dose of 1 g twice daily is safe but does not alter disease course or allow significant reduction of IVIg doses.