Preliminary results of a multicenter, randomized study on the effect of natural interferon beta (n-INFN-beta) in relapsing-remitting MS are reported. Sixty patients received either no treatment or n-IFN-beta (9 MIU three times a week, subcutaneously). After 6 months, the n-IFN-beta-treated group showed a 58% median reduction in the number of active lesions detected on monthly magnetic resonance imaging scans as well as a 38% reduction in the exacerbation rate as compared with the control group. Side-effects were mild and self-limiting. The study continues according to the protocol.