There is pathological evidence that metaplasia of the gastric mucosa to an intestinal type of epithelium is a precancerous condition. First, its incidence and extent are greater in cancerous than noncancerous stomachs. Second, the histological transition between metaplastic mucosa and carcinoma can often be seen in early cancers of the stomach. Third, it would appear that some gastric polyps are composed of intestinal epithelium and probably arise from areas of intestinal metaplasia. Also, recent histochemical evidence is described which supports a relationship between intestinal metaplasia and gastric cancer. The implications of this thesis on the relationship between gastric carcinoma and chronic gastric ulcer, Addisonian anemia and chronic gastritis are discussed. The histology of the gastrointestinal polyps of the Peutz-Jeghers syndrome is described and the opinion given that they are hamartomas and not neoplasms. This, as well as clinical evidence, suggests that this syndrome is not a precancerous condition.