Intestinal metaplasia is defined as the appearance of intestinal epithelium in the stomach. Intestinal metaplasia is frequently found in populations with a high incidence of gastric cancer. Macroscopic demonstration of sucrase and trehalase with Tes-tape in many resected stomachs yielded new information for understanding the nature of intestinal metaplasia. Intestinal metaplasia can be classified into two types, complete and incomplete. The former is associated with the presence of sucrase, trehalase, leucine aminopeptidase, alkaline phosphatase, goblet cells and Paneth cells, and the latter with that of sucrase, leucine aminopeptidase and goblet cells, but not trehalase or Paneth cells. Goblet cells in the complete type of intestinal metaplasia contain sialomucin, as does the small intestine, while those in the incomplete type contain sulphomucin and sialomucin, as does the large intestine. Well-differentiated adenocarcinoma is closely related to intestinal metaplasia, especially the incomplete type. Atypical epithelium of intestinal metaplasia has been proposed as a more proximate stage of gastric cancer. Intestinal metaplasia can be diagnosed by staining with dye under endoscopic observation. A reduced level of pepsinogen I in the blood reflects the presence of severe intestinal metaplasia, which is understood to be a sign of high risk of gastric cancer. Intestinal metaplasia is supposed to be produced by components of food. Mutagens/carcinogens such as N-methyl-N'-nitro-soguanidine and N-propyl-N'-nitro-N-nitrosoguanidine can produce intestinal metaplasia in the glandular stomach of rats and gastric cancers. The formation of intestinal metaplasia precedes the appearance of adenocarcinoma in the glandular stomach. Intestinal metaplasia, which is a kind of host reaction to environmental agents, may result either from genetic change - change in DNA structure - or from epigenetic change - change in the differentiation mechanism. Preventive measures could be developed to suppress the development of intestinal metaplasia and to suppress the process of conversion of metaplastic cells to cancer cells.