It has now been well established that the ulcerogenic syndrome associated with non-beta islet cell tumors of the pancreas is due to excess gastrin release and consequent marked gastric acid hypersecretion. The clinical manifestations may be similar to, but are often more severe and recurrent than, common peptic ulcer. The diagnosis of gastrinoma in patients with this clinical syndrome can be established principally by demonstration of fasting hypergastrinemia, and by application of provocative tests with measurement of serum gastrin in response to intravenous calcium infusion, intravenous secretin injection and feeding of a standard meal. Gastrinomas are usually located within the pancreas, are often multifocal and metastatic, but may be primary in non-pancreatic sites. There is substantial heterogeneity in the molecular forms of circulating and gastrinoma gastrin. Although hypergastrinemia is characteristic of gastrinoma, serum gastrin levels may be increased in disorders other than gastrinoma. Techniques are available to document the presence of gastrin in islet cell tumors and, thereby, to establish these as gastrinomas.