The mutation of p53 is the most common genetic alteration found in human cancers and has oncogenic properties. mdm-2 is a recently discoverd that controls the p53 activity by binding of its protein, so negative feedback loop has been suggested in which p53 induces mdm-2 expression. The purpose of this study was to analyze the expression of p53 in leukoplakias, mdm-2 in squamous cell carcinomas, and relationship between p53 and mdm-2 expression in leukoplakias and squamous cell carcinomas. The results were as follows : 1) The p53 was expressed 33.4% in leukoplakias 2) The mdm-2 was expressed 8.3% in leukoplakias and 22.7% in squamous cell carcinomas. 3) The expression rate of p53 was higher in specimens negative for mdm-2 than in specimens positive for mdm-2, but there was not significant relationship between p53 and mdm-2 expression. In conclusion p53 was thought to participate in early phase of oncogenesis, and mdm-2 was thought to have a role as a oncogene in squamous cell carcinoma of head and neck. Though there was not significant relationship between p53 and mdm-2 expression, mdm-2 was thought to inhibit p53 activity.