The administration of the currently available H2-blockers (at a dosage that induces only partial inhibition of the intragastric acidity) is effective in nearly all peptic ulcer patients in the short and long- term treatment. The benefits of more profound gastric acid inhibition (as achieved with omeprazole) in the short-term treatment of acid peptic diseases has been demonstrated in clinical studies. However, gastric acid has an important physiological role and the potential consequences of profound inhibition of gastric acid include intragastric bacterial colonization and hypergastrinaemia. Bacterial overgrowth of the stomach renders the gut more susceptible to enteric infection and another possible sequela of intragastric bacteria is the formation of N-nitroso compounds with carcinogenic potency. Hypergastrinaemia has a trofic effect on the gastric mucosa and gastric endocrine cells and, in animal, ECL cell hyperplasia and carcinoid formation has been observed as a result of high serum gastrin levels. So far, these potential risks have precluded the long-term administration of omeprazole.