Gastrin is a well known endogenous stimulator of gastric acid. In addition, recent studies have revealed that gastrin has a growth promoting effect on gastric ECL cells. Indeed, development of ECL carcinoid tumor occurs almost exclusively in patients with hypergastrinemia such as autoimmune gastritis and Zollinger-Ellison syndrome with MEN type I. We have recently cloned human gastrin receptor gene, and by using it, we found that both gastric carcinoid tumor and endocrine cell carcinoma of the stomach express significant amount of gastrin receptor gene whereas none of gastric cancer tissue shows gastrin receptor gene expression. Thus, it is clear that gastrin plays important roles in the development of gastric carcinoid tumor as well as endocrine cell carcinoma of the stomach.