Serum MMP3 levels in RA patients with moderate and severe disease activity were higher than in patients with stable RA. Levels in mild RA were not different from those in the stable RA and the healthy control group. Levels decreased in patients treated with certolizumab pegol, a tumor necrosis factor-α (TNFα) inhibitor drug for the treatment of RA. The findings suggest MMP3 as a useful serum biomarker for RA. Here, TNF is linked to rheumatoid arthritis.