We present a novel concept on carcinogenesis mediated by oral microbiota. Oral micro-organisms are capable of metabolizing alcohol to acetaldehyde. This finding casts light on the observed association between poor oral hygiene and oral cancer. Ethanol, as such, is not carcinogenic, but its first metabolite acetaldehyde is indisputably carcinogenic. Several gastro-intestinal microbial species possess the enzyme alcohol dehydrogenase (ADH), which is also the enzyme responsible for alcohol metabolism in the liver. In oral microbiota, we observed that species such as the ubiquitous viridans streptococci and Candida also possess ADH. Ethanol can be detected in the mouth hours after the consumption of alcoholic beverages. Patients with poor oral health status have shown higher salivary acetaldehyde concentrations than those with better oral health. It is thus understandable that ADH-containing micro-organisms in the mouth present a risk for carcinogenic acetaldehyde production, with subsequent potential for the development of oral cancer, particularly among heavy drinkers. In this article, we briefly review this area of investigation and conclude by highlighting some future possibilities for the control of carcinogenesis.