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The matrix metalloproteinases (MMPs) participate in a wide variety of extracellular matrix degradation. Detailed knowledge of MMPs may be important for understanding the pathogenesis of pulpal inflammation. The purpose of this study was to compare MMP-9 expression in clinically healthy human pulp and inflamed human pulp tissue specimens. We compared the levels of MMP-9 between clinically healthy pulp and inflamed pulp tissues by using the semi-quantitative reverse-transcriptase polymerase chain reaction analysis. In addition, immunohistochemistry was used to identify the in situ localization of MMP-9 expression in pulp specimens. MMP-9 mRNA gene was found to be increased in inflamed pulps as compared with clinically healthy pulp tissues (p < 0.05). The results from immunohistochemistry demonstrated that MMP-9 expression was significantly higher in the inflamed pulps than clinically healthy pulps (p < 0.05). MMP-9 stain was detected in the odontoblasts, fibroblasts, inflammatory infiltrates, and endothelial cells. Taken together, MMP-9 may play an important role in the pathogenesis of pulpal inflammation.