Doctors Noseworthy and Summerfield have reviewed the results of placebo-controlled, double-masked, clinical trials of interferon beta-1b (IFNB-1b, Betaseron) and interferons beta-1a (IFNB-1a, Avonex,) in patients with relapsing multiple sclerosis (MS). IFNB-1b, administered subcutaneously every other day to ambulatory patients with relapsing-remitting MS, significantly reduces annual exacerbation rate and percentage change from baseline MRI T2-weighted total lesion area. There is also evidence that IFNB-1b reduces the number of new gadolinium-enhancing lesions in patients with relapsing-remitting MS. IFNB-1a (Avonex), administered intramuscularly every week to patients with relapsing MS, significantly reduces annual exacerbation rate, the number and volume of new focal gadolinium-enhanced T1-weighted lesions, and slows the accumulation of physical disability over time. However, questions remain regarding the efficacy of these treatments, the significance of neutralizing antibody formation, indications for therapy, adverse events, optimal dose, and route of administration. Design limitations of the Phase III trial of IFNB-1b have been reviewed elsewhere, and a critical review of the results of the Phase III trial of IFNB-1a should be undertaken after the publication of the clinical and imaging results of that study.