142 cases of gastric cancer, 20 cases of chronic ulcer, 800 gastrobiopsies, performed for chronic gastritis, were studied morphologically. Adenocarcinoma and undifferentiated gastric cancer were found to have different precancerous lesions in the background. It was shown that among undifferentiated cancers the signs of chronic ulcer preceding cancer are noted more frequently than among adenocarcinomas. Grave forms of atrophic gastritis and gastritis of the mucous membrane regeneration beyond the tumor were revealed more frequently in adenocarcinomas than in undifferentiated cancers. These precancerous affections were found to be associated with the proliferation of different cell elements of gastric mucous membrane, that conditions various listogenesis of basic histological forms of cancer of the stomach. The proliferation of the lining-gastric pit epithelium, mainly observed in gastritis, results in the appearance of adenocarcinomas. The proliferation of the glandular epithelium, particularly that of glandular ducts in ulcerous disease would condition the development of undifferentiated forms of gastric cancer.