We analyzed an acetaldehyde-derived DNA adduct, N(2)-ethylidene-2'-deoxyguanosine (N(2)-Eti-dG) in stomach DNA of aldehyde dehydrogenase (Aldh)-2-knockout mice that were fed with alcohol to determine effects of alcohol consumption and Aldh2 genotype on the level of DNA damage in stomach. Aldh2-active(+/+), heterozygote(+/-) and knockout(-/-) mice were fed 20% ethanol for 5 weeks, then the level of N(2)-Eti-dG in stomach was determined by liquid chromatography tandem mass spectrometry. The average N(2)-Eti-dG level in DNA from untreated mice was not significantly different among Aldh2 genotypes (2.0-3.1 adducts/10(7) bases), however, the average N(2)-Eti-dG level in DNA from ethanol-treated mice was 4.8+/-2.6 adducts/10(7) bases in Aldh2+/+ mice, 7.9+/-1.1 adducts/10(7) bases in Aldh2+/- mice, and 48.6+/-12.0 adducts/10(7) bases in Aldh2-/- mice, respectively. Our data clearly showed that alcohol drinking caused DNA damage in stomach, which was Aldh2 genotype-dependent in this experimental animal model. This result suggests that heavy-alcohol drinking and Aldh2 deficiency might be risk factors of stomach cancer.