MMP-2 and MMP-9, type IV collagenases which degrade basement membrane, have been known to play important roles in invasion and metastasis of tumor cells, In addition, they seem to be involved in cell differentiation, apoptosis, angiogenesis and immunity, etc. We immunohistochemically examined epithelial and stromal expressions of MMP-2 and MMP-9 in irritation fibroma, oral leukoplakia, and oral squamous cell carcinoma (OSCC) and have some results as follows: 1. Irritation fibromas, oral leukoplakias and OSCCs mostly showed increased expression of MMP-2 and MMP-9 in the epithelium and connective tissue compared with normal mucosa. 2. There was a significant difference in the epithelial expression of MMP-2 and MMP-9 between irritation fibroma and oral leukoplakia. 3. There was a significant difference in the epithelial and stromal expression of MMP-2 and MMP-9 between irritation fibroma and OSCC. 4. There was a significant difference in the stromal expression of MMP-9 between oral leukoplakia and OSCC. We concluded that rritation fibroma, oral leukoplakia and OSCC have somewhat different characteristics of MMP-2 and MMP-9 expressions, which perhaps result from different pathogenesis.