Six cases of duodenal ulcer were studied in male children (mean age 8.8 yrs., range 6-12 yrs., mean weight 29.8 kg). All were submitted to X-ray examination and/or digestive endoscopy. Ten healthy children, matched by weight, age and sex were chosen as controls. All children underwent a standardized protein meal to evaluate serum gastrin and pepsinogen I response and the pentagastrin test for acid secretion. The serum gastrin level was found to be similar in the two groups (normal children and duodenal ulcer) both in the fasting state and after food stimulation, whereas the basal and after-meal serum pepsinogen I values were statistically higher in the duodenal ulcer group (p less than 0.01). The pentagastrin test showed a basal, maximal and peak acid output significantly lower in controls than in the subjects with primary duodenal ulcer. The results confirm that elevated gastric acid response is already present in duodenal ulcer of childhood and seems to be its cause rather than its consequence. Our finding of an already elevated pepsinogen I level, coupled with the already reported family histories of the disease, further support an inherited basis for duodenal ulcer in childhood.