OBJECTIVE To evaluate the efficacy and safety of IFNbeta-1a (Avonex, Biogen, Inc., Cambridge, MA, USA) in patients with relapsing-remitting multiple sclerosis (MS). METHODS In this multicenter, open-label, prospective clinical trial, 96 patients with relapsing-remitting MS received IFNbeta-1a 30 mcg intramuscularly once weekly for 2 years. Outcome variables included: change from baseline in mean number of exacerbations, proportion of exacerbation-free patients, and mean Expanded Disability Status Scale (EDSS) scores at Years 1 and 2. RESULTS IFNbeta-1a significantly (P < 0.0001) reduced exacerbation rate at Years 1 and 2 of treatment. The percentage of exacerbation-free patients was 53% during Year 1 and 33% during Year 2. Mean EDSS scores were 2.96 +/- 1.26 at baseline, 2.89 +/- 1.42 at Year 1, and 3.00 +/- 1.62 at Year 2 (P = 0.116). EDSS scores improved in 35.4%, remained stable in 28.1%, and worsened in 36.5% of patients. IFNbeta-1a treatment was well tolerated. CONCLUSION This study confirms and extends the beneficial clinical profile for IFNbeta-1a in relapsing MS.