We analyzed environmental factors, family history of peptic ulcer, gastric acid secretion, and serum levels of pepsinogen I (PG I) and gastrin in 56 juvenile patients with duodenal ulcer and 39 normal teenage subjects. Basal acid output and maximal acid output were significantly higher in our duodenal ulcer patients than in controls without ulcer (both, p < 0.01), and patients with duodenal ulcer showed significantly higher serum levels of PG I and gastrin than the controls (both, p < 0.001). There were no significant differences in any environmental factor between the patients and controls. Fifteen of the 17 patients who had one or both parents with hyperpepsinogenemia I had high serum PG I levels. Over half of the duodenal ulcer patients had high serum gastrin levels, irrespective of family history of hypergastrinemia. Our findings suggest that hyperpepsinogenemia I and hypergastrinemia are important characteristics and that genetic background, particularly the inheritance of a gastric mucosal trait expressed as hyperpepsinogenemia I, is frequently involved in the pathogenesis of juvenile duodenal ulcer.