INTRODUCTION: Multiple sclerosis (MS) is recognised as a central nervous system disease also affecting cognition. The rate of cognitive dysfunction in MS is in the range of 45-65% and adversely affect the quality of life. OBJECTIVE: To evaluate the effect of 1 year of treatment with interferon-beta-1b (IFNbeta-1b) on cognitive functions in patients suffering from relapsing-remitting MS. METHODS: A battery of cognitive tests was used to assess verbal learning, delayed recall, visual learning and recall, complex attention, concentration and verbal fluency at baseline and after 1 year of treatment with IFNbeta-1b. A group of 23 relapsing-remitting MS patients matched for neurological disability served as controls. RESULTS: Eighteen of 23 patients treated with IFNbeta-1b (74%) completed the study. In the IFNbeta-1b-treated group, complex attention, concentration as well as visual learning and recall improved significantly (p = 0.024, p = 0.006 and p = 0.005, respectively), while no deterioration was observed in the other dimensions. In the control group, complex attention, verbal fluency, as well as visual learning and recall deteriorated significantly (p = 0.02, p = 0.004 and p = 0.01, respectively), while no deterioration was observed in the other dimensions. CONCLUSION: Immunomodulating drugs that reduce the relapse rate and slow the disease progression also inhibit cognitive deterioration in patients with MS.