In this paper the authors focus attention on the role of gastrin as a carcinogenic factor. The aim was to bring together the numerous controversial studies on this subject, adding the authors' own personal clinical experience. Gastrin (G) is responsible for the development of carcinoids, as has been experimentally shown in Mastomys rats, and more recently in man. This hormone is regarded as a mitogen for cells in the gastroenteric tract; it acts through specific reactors and messengers, including AMPc and protein kinase A (PKA). Its role in the development of other neoplasias of the gastroenteric tract appears to be linked, but not always subordinate, to the presence of growth factors such as: EGF and TGF-alpha, and also to the possible stimulation of oncogens induced by hypergastrinemia.