The p53 mutations in betel-related oral cancer have been studied but the results may be confounded by tobacco and alcohol due to tobacco components in the quid or concomitant habits of smoking and drinking. In this study, p53 mutations in tumors from Thai betel chewers who neither smoke nor drink were studied in comparison to tumors from smokers/drinkers. PCR-SSCP analysis and direct sequencing on exon 5-8 were performed. Mutations were detected in 11.8% (8/68) of betel-related tumors and 7 of 8 mutations were G:C to A:T transitions. By contrast, mutations were found in 22.4% (13/58) of smoking/drinking-related tumors with various base substitutions. The results demonstrated different mutation profiles between the two exposed groups. The type of mutation detected in betel chewers suggested a possible role of areca-specific nitrosamine as a causative carcinogen.