Omeprazole, a substituted benzimidazole, is a specific inhibitor of the enzyme H+/K(+)-ATPase, which is found on the secretory surface of the parietal cell. This enzyme, the "proton pump," catalyzes the final step in acid secretion. Omeprazole is a powerful inhibitor of gastric acid secretion. At the time of writing, omeprazole has been licensed in the United States for the treatment of severe grades of gastroesophageal reflux disease (GERD) as well as GERD unresponsive to treatment with currently available agents, and for the treatment of Zollinger-Ellison syndrome and other gastric hypersecretory states. Most recently, it has been recommended by the FDA advisory committee for approval as first-line therapy in duodenal ulcer disease.