Objective: To investigate the pathogenesis of cavity adhesions by studying the expression of MMP-9 in endometrial tissues. METHODS: Thirty patients in the observation group and 30 patients in the non-adhesion group were prospectively studied. The expression levels of matrix metalloproteinase-9 (MMP-9) in the endometrial tissues of both groups were determined by semi-quantitative immunohistochemistry. Results: The expression of MMP-9 in the endometrium of the observation group was significantly lower than that of the control group, and there was a highly significant difference in the expression of MMP-9 in the endometrium of the two groups (P = 01000 < 0101); in the observation group, the expression of MMP-9 in the endometrium of patients with severe cavity adhesions was significantly lower than that of patients with mild and moderate cavity adhesions, and there was also a significant difference in the expression of MMP-9 in the endometrium of the two groups (P < 0105). Conclusion: The abnormal expression of MMP-9 in endometrial tissue may be involved in the formation of cavity adhesions.