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To investigate the value of metalloproteinase-3 (MMP-3) levels in assessing efficacy of etanercept treatment in patients with ankylosing spondylitis (AS).The serum and synovial fluid levels of MMP-3 were measured by enzyme linked immunosorbent assay (ELISA) in 48 patients with AS in week 0, 6 and 12; and also measured in 30 serum samples and 10 synovial fluid samples from healthy controls.The serum levels of MMP-3 in AS patients were significantly higher than those in controls. In AS patients, the MMP-3 levels in synovial fluid were significantly higher than those in serum levels. The serum MMP-3 levels in AS patients with peripheral arthritis were higher than those with exclusively axial involvement; while C-reactive protein (CRP) levels and erythrocyte sedimentation rate (ESR) did not differ between these 2 groups of AS. At week 6 and week 12 of etanercept treatment, the serum MMP-3 levels were significantly decreased (p0.01) with the declining trend of ESR, CRP, Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and Bath Ankylosing Spondylitis Functional Index (BASFI) (all p0.01). Before the etanercept treatment (week 0), serum levels of MMP-3 were correlated with ESR, CRP, BASDAI and BASFI (p0.05). ESR was also correlated with CRP and BASFI, but not with BASDAI (r=0.361, P=0.071). At weeks 12, serum MMP-3 levels were still correlated with ESR, CRP and BASDAI (P0.05), but not with BASFI (P=0.339); ESR was correlated with CRP, but not with BASDAI and BASFI. There was a significant correlation between BASDAI and BASFI (r=0.818,P=0.001).Serum MMP-3 levels are closely related to disease activity and may serve as an useful indicator for efficacy of etanercept treatment in AS patients.