BACKGROUND Acetaldehyde, the first metabolite of alcohol, has been proposed to be the carcinogenic substance behind ethanol-related oral cancers. High levels of acetaldehyde are formed from ethanol in saliva by the oral flora, but so far the role of certain microbial species responsible for this phenomenon is not known. Yeasts are common commensals of the oral cavity that have alcohol-oxidizing enzymes, thus providing a potential source of acetaldehyde from ethanol. The aim of this study was to examine the contribution of oral yeasts to the production of ethanol-derived acetaldehyde in the oral cavity. METHODS Fifty-five saliva samples were divided into two groups, high and low, based on the in vitro salivary acetaldehyde production capacity from ethanol. Yeasts were isolated and identified from these samples, and their acetaldehyde production capacity was determined gas chromatographically by incubating intact cells with ethanol at the physiological pH of 7.4. RESULTS Yeast colonization was found in 78% of the high acetaldehyde-producing salivas, compared with 47% in the low acetaldehyde-producing salivas (p = 0.026). Among carriers, the density of yeasts was higher in the high than in low acetaldehyde producers (p = 0.025). Candida albicans was the main species isolated (88% of all oral isolates). Moreover, C. albicans strains isolated from the high acetaldehyde-producing salivas formed significantly higher acetaldehyde levels from ethanol than C. albicans strains from low-acetaldehyde-producing salivas (73.1 nmol ach/10e6 colony-forming units vs. 43.2 nmol ach/10e6 colony-forming units, p = 0.035). CONCLUSIONS This study shows that some C. albicans strains have a marked capacity to produce toxic and carcinogenic acetaldehyde from ethanol in vitro. Because the in vitro production of salivary acetaldehyde has been previously shown to correlate with in vivo acetaldehyde production, our finding could be an important microbial pathogenetic factor underlying cancer of the oral cavity associated with ethanol drinking.