Objective To determine the correlations of metabolic syndrome (MS) with mild cognitive impairment (MCI) in the elderly, and investigate the role of matrix metalloproteinase 9 (MMP-9) activity in the process. Methods A total of 135 MS patients [MS group, with an age ranging from 65 to 76 (67.8±6.1) years] and 75 healthy control subjects [Control group, with an age ranging from 65 to 75 (66.3±5.9) years] admitted in our department from March 2009 to June 2012 were enrolled in this study. Mini-mental status examination (MMSE) was applied to evaluate their cognitive function. The plasma levels of MMP-9 and tissue inhibitor of metalloproteinase 1 (TIMP-1) were measured to estimate MMP-9 activity by calculating the ratio of MMP-9 to TIMP-1. Results MCI was more common in MS group than in control group (22.2% vs 9.33%, P＜0.05). The scores of MMSE were significantly lower in MS group than in control group [(26.12±1.15) vs (29.37±0.58), P＜0.05]. MS group had significant higher MMP-9 activity than the control group [(0.81±0.35) vs (0.59±0.27), P＜0.05]. In MS group, the MMP-9 activity was negatively correlated with MMSE score (r＝-0.519, P＜0.05). A high level of education was a protective factor of MCI (OR＝0.615, P＝0.023), while MS, diabetes and MMP-9 activity were independent risk factors of MCI (OR1＝6.981, P1＝0.011; OR2＝6.250, P2＝0.017; OR3＝3.262, P3＝0.031). Conclusion MS is an independent risk factor of MCI. In MS patients, MMP-9 activity is positively correlated with the severity of MCI.