We compared the relative efficacy of interferon beta (IFNbeta) products and azathioprine (AZA) in the treatment of relapsing- remitting multiple sclerosis (RRMS). Ninety-four previously untreated patients of short duration with RRMS were randomly allocated to the two treatment groups. The first group received IFNbeta products (Betaferon,Avonex or Rebif); the second group received AZA for 12 months. Response to treatment was assessed at 3, 6, and 12 months after starting therapy. The mean number of relapse during one year of the study was lower in the AZA group than in the IFNbeta products group (0.28 vs. 0.64, P < 0.05). After 12 months, 57.4% of patients receiving IFNbeta products remained relapse free compared with 76.6% of those given AZA. The Expanded Disability Status Scale (EDSS) decreased by 0.30 units in IFNbeta-treated patients (P < 0.05) and 0.46 in AZAtreated patients (P < 0.001). Treatment with IFNbeta products and AZA significantly reduces the relapse rate and EDSS score in patients with RRMS, while AZA is more effective than the IFNbeta formulations.