The purpose of these controlled studies was to determine the prevalence of acid-pepsinogen hypersecretion in 173 patients with duodenal ulcer disease [88 Americans (75 men, 13 women) and 85 Chinese (66 men, 19 women)]. One-half to two-thirds of duodenal ulcer patients of either sex had acid hypersecretion or hyperpepsinogenemia, or both. When Chinese and American duodenal ulcer patients were compared, the two ethnic groups had similar serum pepsinogen I and II concentrations and similar maximal acid outputs per kilogram body weight. In contrast, Chinese duodenal ulcer patients had significantly lower basal acid outputs per kilogram body weight than American duodenal ulcer patients. We conclude that acid-pepsinogen hypersecretion is present in the majority of American and oriental duodenal ulcer patients.