Serum pepsinogen 1, serum gastrin, ABO blood groups, secretor status of ABH blood group substances and behavioral factors were studied in 15 patients with duodenal ulcer and 61 their relatives affected and unaffected to duodenal ulcer. Duodenal ulcer patients had hyperpepsinogenemia 1 either with or without a positive family history of duodenal ulcer. Serum gastrin level was higher in duodenal ulcer patients and unaffected relatives with a positive family history of duodenal ulcer than those with a negative family history. Non secretor status was frequently observed in duodenal ulcer patients with a positive family history. There was no difference in behavioral factors between duodenal ulcer patients and unaffected relatives with a positive family history. It is concluded that genetically determined variables such as hyperpepsinogenemia 1 and non secretor status play an important role on the susceptibility to duodenal ulcer in subjects with a positive family history, and hypergastrinemia may be subclinical marker of familial aggregation of duodenal ulcer.