Helicobacter pylori is known to infect a half of the world's population and has been closely linked to an increased risk of the development of gastric adenocarcinoma, making it a pathogen of potentially great significance. The IARC/WHO in 1994 designated H. pylori a class I carcinogen based on epidemiological evidence. A direct association between H. pylori infection and the induction of gastric carcinoma has been recently demonstrated in a Mongolian gerbil model with use of chemical carcinogens, giving further credence to the role of this organism as a promoter in gastric carcinogenesis. However, the exact mechanisms underlying the link between H. pylori infection and gastric carcinogenesis still remain to be elucidated. To approach this issue, it is necessary to find environmental factors and to clarify genetic events during carcinogenesis in in vivo models.