Epidermal growth factor receptor has been reported to be overexpressed in head and neck squamous cell carcinoma and terefore it could be used as a tumor marker. In situ mRNA hybridization, that is known to be more specific and sensitive than traditional biochemical and immunohistochemical studies has been rarely used for EGFR expression. In this study, author performed both in situ mRNA hybridization and immunohistochemical stain for EGFR in 51 squamous cell carcinoma of the oral cavity to compare the expression rates of EGFR by these two methods and evaluated the correlation between their expression rates and clinical data such as subsites, primary tumor stages, neck node metastasis, clinical stages, cancer recurrance, treatment results, and pathologic grades. The results were as follows : 1) EGFR was positive in 49 cases(96.1%) by in situ mRNA hybridization and in 37 cases(72.5%) by immunohistochemical stain. 2) The expression was stronger in the more histologically differentiated cells by in situ mRNA hybridization of EGFR. 3) There was no correlation between the expression of EGFR by in situ mRNA hybridization and several factors including primary tumor stages, clinical stages, neck node metastasis and treatment results. 4) Three positive expression was observed in all recurred cases by in situ mRNA hybridization of EGFR.