IFN-beta treatment is emerging as a potentially effective form of therapy in various immune-mediated conditions. The present review addresses the possible role of IFN-beta in immune-mediated diseases such as multiple sclerosis and rheumatoid arthritis. Several placebo-controlled trials are discussed, as are the available immunological data that are relevant to this field. Review of these data provides evidence that IFN-beta has some beneficial therapeutic effect in patients with relapsing-remitting multiple sclerosis and might also have antirheumatic potential. This notion is supported by recent studies showing a critical role for IFN-beta in bone homeostasis.