There is extensive evidence that immune mechanisms are involved in the pathogenesis of multiple sclerosis (MS). A potential treatment effect of IVIg in MS could be mediated by several mechanisms as IVIg is able to modulate the immune response in different ways. The evidence of the effect of IVIg in MS is at present based on the results of small open trials, some of which have been encouraging. Confirmation of a beneficial effect of IVIg must await the results of placebo-controlled, double-blind trials. A placebo-controlled, crossover study of IVIg is being conducted using magnetic resonance imaging (MRI) for monitoring disease activity. However, the ultimate proof of efficacy must come from studies of parallel groups with large numbers of patients followed for long periods with repeated clinical observations of neurological function, for example, the Expanded Disability Status Scale (EDSS) or the Neurological Rating Scale (NRS).