Two standard Helicobacter pylori strains showed significant cytosolic alcohol dehydrogenase activity and produced considerable amounts of acetaldehyde when incubated with an ethanol containing solution in vitro. The alcohol dehydrogenase activity of the Helicobacter pylori strains was almost as high as that found in Klebsiella pneumoniae and far greater than that in Escherichia coli or Campylobacter jejuni. The amount of acetaldehyde produced by cytosol prepared from Helicobacter pylori exceeded that by any of the other bacteria studied. The bacterial production of acetaldehyde--a highly toxic and reactive substance--could be an important factor in the pathogenesis of Helicobacter pylori associated gastric injury and increased risk of gastric cancer.