Matrix metalloproteinase-3 is abundantly expressed in active rheumatoid synovium, and serum level of MMP-3 is a useful marker not only for the diagnosis of rheumatoid arthritis but also for the evaluation of prognosis in the joint destruction. It should be noted, however, that serum MMP-3 may be increased in other inflammatory arthritis, thus its specificity for the diagnosis of early RA may not be high enough. Recent reports indicated that methotrexate or anti-TNF alpha therapy effectively suppressed the serum MMP-3 level during the course of the treatment, whereas corticosteroid didn't. Thus, suppression of serum MMP-3 could explain the discrepancy in the inhibition of bony destruction in RA patients by the therapeutic strategy.