The product of the adenomatous polyposis coli (APC) tumor suppressor gene has been observed to regulate the Wnt signaling pathway through beta-catenin. In the present study, we attempted to clarify the relation between APC and the canceration of oral squamous epithelium. Each target tissue of normal squamous epithelium, epithelial dysplasia, and squamous cell carcinoma (SCC) was recovered the oral SCC case by laser microdissection. In recovered cells, we examined the change in expression of APC and beta-catenin gene transcription products, as well as the existence of mutations in APC gene. We analyzed the localization of each protein of APC and beta-catenin by immunohistochemical study. We found a clear change in the expression level of the gene transcription product of APC in canceration of oral squamous epithelium and the differentiation of oral SCC. In addition, there was some change in the localization of the APC protein in canceration. It was not clear, however, whether the APC was mutated. A change was also observed in the expression level of the beta-catenin gene transcription product during the differentiation of oral SCC. Our results suggest that the changes in the expression level and the intracellular localization of APC are related to the canceration of oral squamous epithelium, and in malignant characterization of oral SCC. Mutations of the APC gene might not be indispensable, however, in canceration of oral squamous epithelium.