The frequency of stomach cancer in females is lower than that in males, but the cancers in females are often Borrmann type 4 or poorly differentiated adenocarcinomas. In young patients, the frequency of gastric cancer in females is higher than that in males, and widely spread cancers are often observed just after pregnancy and delivery. The incidence of stomach cancer in female rats is lower than that in males but the incidence increased slightly after pregnancy and delivery. And it decreased when female sex hormones are injected to male rats. It is surmised that female sex hormones or their analogues play a suppressive role on carcinogenesis or progression of stomach cancer, and that pregnancy and delivery may accelerate the growing of stomach cancer. Hormone therapy was performed to get some effects, and to take much more effects, some other new trials on hormone therapy are necessary.