Recent studies have suggested that abnormalities in the adenomatous polyposis coli gene (APC gene) are associated with the development not only of familial adenomatous polyposis coli (FAP) but also of cancers in digestive organs. In order to elucidate whether abnormalities of the APC gene could contribute to the development of oral squamous-cell carcinoma (SCC), genomic DNAs from tumors and normal tissues of 24 unrelated Japanese patients were examined by using PCR-SSCP (polymerase chain reaction single-strand conformation polymorphism) and sequence analyses. Five novel nucleotide substitutions of the APC gene in tumor tissues were identified in 3 patients with oral SCC (12.5%), resulting in 3 amino-acid replacements or a truncation of the APC gene product. We also examined 24 tumor and 24 normal tissue samples for loss of heterozygosity (LOH) at exon 11 of the APC gene by PCR-LOH assay. In this analysis, 45.8% of samples were informative and LOH was detected in 72.7% of informative cases. The frequency of LOH in oral SCC was similar to that previously reported in esophageal SCC. These results suggest that abnormalities in the APC gene are associated with the development of human oral SCC.