Abstract. PTEN is a tumor suppressor gene located on chromosome10q23.3. In addition to genetic mutations and deletions, the down-regulation of PTEN has been found in various malignant tumors. However, little is known about the profile of PTEN gene in oral carcinomas. In this study, the expression profiles and genetic alterations of PTEN were examined in 113 oral squamous cell carcinoma (OSCC) cases and 9 OSCC-derived cell lines. An immunohistochemical analysis showed statistically significant differences in the immunohistochemical (IHC) scores for PTEN protein in normal tissues and in cancerous regions (P=0.0104), suggesting that PTEN protein expression is down-regulated in OSCC. No significant correlations existed between the down-regulation of PTEN protein expression and the clinicopathological features of the tumor. Reverse transcriptionpolymerase chain reaction (RT-PCR) analysis showed a lower PTEN mRNA expression in the 9 OSCC-derived cell lines examined, as compared to the normal oral epithelium cells. However, treatment with a demethylating reagent restored PTEN mRNA expression in 4 cell lines. No genetic mutations were detected in these cell lines by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. The results suggest that epigenetic changes may be related to the down-regulation of PTEN expression. We therefore conclude that PTEN is a crucial molecule in the tumorigenesis of OSCC.