Objective: To investigate the concentration of serum Matrix Metalloproteinase (MMP)-3 and -9 in active ankylosing spondylitis (AS) during infliximab treatment and to evaluate whether MMP-3 and MMP-9 were associated with clinical measures of disease activity and function in AS. Method: Baseline and sequential serum MMP-3 and MMP-9 were examined at week 12 of infliximab treatment in 44 AS patients. The clinical sores were evaluated by ASDAS, BASFI, BASMI, ESR and CRP. The paired T-test and Spearman correlation analysis were used for statistic analysis. Results: (1) Serum MMP-3 and MMP-9 and clinical scores including ASDAS, BASFI, BASMI, ESR and CRP of 44 AS patients significantly decreased when they were treated with infliximab for 12 weeks (P<0.05). (2) AS with peripheral arthritis (PA) patients had higher MMP-3 level than AS without PA patients (P<0.01). Contrastly, AS without PA patients had higher MMP-9 level (P<0.05). (3) The association was found between MMP-3 and ESR, CRP, ASDAS and BASFI (P<0.01), and MMP-9 was associated with CRP, ASDAS, BASFI and BASMI (P<0.05). (4) After 12 weeks, the decrease of MMP-3 was associated with the improvement of ASDAS, BASFI, ESR, CRP, but not with BASMI. The decline of MMP-9 was correlated with the improvement of ASDAS, BASMI and CRP. (5) There were thirty-seven responders and seven non-responders according to the improvement of ASDAS criteria at week 12 after infliximab infusion. Responders have higher baseline serum MMP-3 than that of non-responders (P<0.01), but not MMP-9. Conclusions: Serum MMP-3 and MMP-9 decreased significantly in active AS patients during infliximab therapy and their decline was associated with the improvement of clinical syndromes. Infliximab responders have higher baseline serum MMP-3 not MMP-9 level than that in infliximab non-responders.