We performed a post-marketing study of patients with multiple sclerosis (MS) attending the outpatient service to evaluate the impact of interferon beta-1b (IFNbeta-1b) in the daily clinical setting. The absolute changes in relapse frequency and in the mean EDSS score over a three-year period were compared between 83 patients with relapsing remitting MS treated with IFNbeta-1b and 83 RRMS patients who did not take the drug. Annualized relapse frequency significantly decreased in patients undergoing therapy while no statistically significant changes in EDSS score were observed. These findings point out the role of post-marketing studies in evaluating the impact of approved drugs in the daily clinical setting in terms of safety and tolerability. Furthermore, our results confirm the positive effect of immunomodulatory treatment in decreasing the occurrence of inflammatory events.