The Zollinger-Ellison syndrome (ZES) is characterized by gastrin-producing tumors, gastric acid hypersecretion, peptic ulcers, and diarrhea. ZES may be associated with nonpancreatic endocrine tumors as part of the multiple endocrine adenomatosis-type 1 syndrome. The diagnosis of ZES can be confirmed in patients with a suggestive clinical picture and excessive gastric acid secretion by the finding of a markedly elevated fasting serum gastrin level or characteristic gastrin response to provocative testing. Standard treatment of ZES has traditionally been surgical. With the introduction of H2-histamine receptor antagonists, however, many patients can now be managed successfully with cimetidine alone, or cimetidine plus vagotomy, when no resectable gastrinoma is present.