Several inteferons (IFNs) have recently been introduced for the treatment of multiple sclerosis (MS). These are IFNbeta-1b (Betaseron) and 2 IFNbeta-1a preparations (Avonex and Rebif). All 3 drugs reduce the number of acute exacerbations in relapsing remitting MS by about one-third but, because of differences in the trials and trial design, direct comparison of efficacy is difficult. Only the IFNbeta-1a preparations have been demonstrated to have an effect in slowing disability. Possible reasons for disparate results with the 3 IFNbetas include substantial differences in trial design, antigenicity, route of administration and bioavailability. Adverse effects differ among the various preparations. Avonex, which is given intramuscularly once weekly, has the fewest adverse effects and the lowest incidence of neutralising antibody formation at 5 to 7%. IFNbeta-1b has the highest incidence at 30 to 39%, and Rebif is intermediate at 13 to 24%. Additionally, IFNbeta-1b and Rebif often cause significant skin reactions, and the former causes occasional skin necrosis at injection sites, a problem not seen with Avonex. On the basis of the available information, the IFNbeta-1a preparations are preferred in terms of tolerability, effects on exacerbation rate and disability, less frequent antibody formation and convenience. The available preparations, Avonex and Rebif, have shown similar effectiveness although dose, route of administration and dosage regimens differ. The administered dose of IFNbeta-1b and Rebif appears to be somewhat higher than that of Avonex, but the differences in effect are small. Further information regarding the effect of different doses and dosage regimens with the various preparations is expected to become available over the next few years.