Objectives: Matrix metalloproteinases (MMPs), particularly MMP-9, facilitate T-cell migration into the central nervous system. They play a key role in the disruption of the blood-brain barrier (BBB) and thus in the pathogenesis of multiple sclerosis. Interferon beta's (IFNβ) ability to alter the balance between MMP-9 and MMP-9s natural inhibitor, tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), may play a role in stabilizing the BBB. The aim of this study, was to evaluate serum MMP-9 and TIMP-1 and cerebrospinal fluid (CSF) TIMP-1 levels in patients with relapsing-remitting multiple sclerosis (RRMS) treated with IFNβ-1a. Patients and methods: Blood and CSF samples from 14 patients with RRMS before and 6 months after IFNβ therapy and 14 age and sex-matched controls were obtained. Levels of MMP-9 and TIMP-1 were measured using ELISA. Results: Before treatment, patients with MS had higher levels of serum MMP-9 and a higher MMP-9/TIMP-1 ratio than the controls. Although serum levels of TIMP-1 were lower in RRMS patients than in the controls, the differences did not reach statistical significance. CSF levels of TIMP-1 were significantly lower in RRMS patients. In the sixth month of IFNβ therapy serum MMP-9 and the MMP-9/TIMP-1 ratio were significantly decreased, whereas the changes in serum TIMP-1 were not statistically significant. There was a significant increase in CSF TIMP-1 levels in the sixth month of IFNβ therapy. Conclusions: Our result shows that RRMS patients have an impaired MMP-9 and TIMP-1 balance, and that 6 months of IFNβ therapy is beneficial in restoring this balance. © 2005 Elsevier B.V. All rights reserved.