We evaluated the incidence and risk factors of duodenal ulcer (DU) in a cohort of patients who had undergone esophagogastroduodenoscopy in our institution from 1978 to 1982, without then finding a gastric or duodenal ulcer. Six hundred and twenty-one subjects entered the study, and 526 (84.7%) were traced in 1992. Forty-one cases of DU were found, an incidence rate of 6.7 per 1,000 person-years; the male/female ratio was 2:1. The diagnoses of peptic ulcer (PU) were validated by means of medical documentation. The odds ratio of DU was 2.3 (95% confidence interval 0.8-6.3) and 3.5 (1.4-9.0) in the II and III tertile of basal acid output (BAO) compared with the I tertile, and 2.6 (0.8-8.3) and 7.0 (2.3-20.7) in the II and III tertile of maximal acid output (MAO) compared with the I tertile. Cigarette smoking and the presence of PU among siblings of DU patients were risk factors for DU. Multiple logistic regression confirmed only the association of DU with MAO, cigarette smoking, and family history. By means of a nested case-control study, the risk of developing DU was assessed in subjects with Helicobacter pylori (HP) at the biopsy of gastric mucosa performed before the ulcer diagnosis. The odds ratio of DU in HP-positive subjects was 5.0 (95% confidence interval 0.6-45). The results of this study confirm the influence of gastric acid secretion, cigarette smoking, and family history in the pathogenesis of DU and suggest that HP infection not only favors relapse but could have an important role in the onset of the disease.