PTEN, a tumor suppressor gene, has been found to be inactivated by structural abnormalities or epigenetic changes in several types of human cancers. Recently, several studies have also suggested the possibility that the PTEN gene is a target of genomic instability in human cancers displaying microsatellite instability (MSI). To investigate the role of PTEN in human oral squamous cell carcinomas, we screened the entire coding region sequences and examined the expression of the PTEN gene in 81 oral cancers displaying microsatellite stability (MSS) and 5 oral cancers displaying MSI. Mutation of the PTEN gene was identified in one MSS cancer (1/81; 1.2%) and three MSI cancers (3/5; 60%). The MSS cancer harbored a missense mutation from Ala (GCA) to Val (GTA) at codon 137. Of the MSI cancers containing the PTEN mutation, case 36 had a missense mutation from Lys (AAA) to Glu (GAA) at codon 254, case 43 contained a frameshift mutation (one A deletion) in a 6 bp poly(A) tract affecting codon 265-267, and case 64 harbored two missense mutations from Val (GTG) to Ala (GCG) at codon 222, and from Gly (GGA) to Arg (AGA) at codon 230 indicating biallelic mutation of PTEN. Genomic deletion of exon 5, resulting in loss of PTEN mRNA, was observed in two MSS cancers. In spite of an intact PTEN gene, one MSS and one MSI cancer lacked PTEN mRNA. These findings suggest that the inactivation of PTEN by either mutation or loss of transcript plays a role in the pathogenesis of some oral cancers (8/86; 9.3%). Furthermore, inactivation of PTEN was far more frequent in MSI oral cancers (4/5; 80%) than in MSS oral cancers (4/81; 4.9%).