Omeprazole, an H+K+ATPase inhibitor, is a potent suppressor of gastric acid secretion. A dose of 30 mg/day caused a 97% decrease of mean 24-hour intragastric acidity in duodenal ulcer patients. Basal and postprandial plasma gastrin concentrations were increased during once daily omeprazole treatment, but were reversed after cessation of treatment. The object of this study was to investigate whether this omeprazole induced elevation of plasma gastrin has a trophic effect on the parietal cell mass in man.