Chronic ethanol ingestion leads to an enhanced risk of upper gastrointestinal tract cancer. Although many hypotheses for the tumor promoting effect of alcohol exist, the pathogenetic mechanisms remain unclear since alcohol in itself is not carcinogenic. Acetaldehyde, the first metabolite of ethanol, has been shown to have multiple mutagenic effects and to be carcinogenic to animals. Previous research has revealed that acetaldehyde can be formed from ethanol via microbial alcohol dehydrogenase. Thus, at least part of the proposed tumorigenic effect of ethanol may be linked to local production of acetaldehyde from ethanol by oral microflora. In this study we demonstrate the production of marked amounts of acetaldehyde in saliva after ingestion of moderate amounts of ethanol. Considerable inter individual variation in acetaldehyde production capacity is also shown. In vivo acetaldehyde production is significantly reduced after a 3-day use of an antiseptic mouthwash (chlorhexidine). In vitro acetaldehyde production was shown to be linear in time, inhibited by 4-methylpyrazole and it could not be saturated under ethanol conditions that are relevant in vivo. There was a significant positive correlation between salivary acetaldehyde production in vitro and in vivo. We conclude, that the microbial formation of acetaldehyde in saliva could be one explanation for the tumor promoting effect of ethanol on the upper gastrointestinal tract. Moreover, this may support the epidemiological finding, that poor oral hygiene is an independent risk factor for oral cavity cancer.