Stromelysin-1 or matrix metalloproteinase-3 (MMP-3) is a key enzyme of the degradation of the extracellular matrix in rheumatoid arthritis. MMP-3 is indeed induced by pro-inflammatory cytokines and may in turn activate other metalloproteinases. MMP-3 is a synovial parameter reflecting the local inflammatory reaction induced by inflammatory cytokines. It is not specific for rheumatoid arthritis nor for the erosive capacity of an arthropathy, which does not exclude its role in articular degradation. Serum MMP-3 is increased in diseases characterized by synovitis but also by steroid therapy. Although it is not disease-specific, serum MMP-3 could be useful as a synovial-derived marker of local inflammation in rheumatoid arthritis, in parallel to CRP which is a systemic marker of inflammation. Furthermore, early determination of serum MMP-3 could constitute a new tool predictive of the disease activity and the therapeutic response in rheumatoid arthritis.