We present 28 children, 4 to 14 year-old, with duodenal ulcer; there were 21 males and 7 females. In 16 cases, after stimulation with pentagastrin, basal pepsinogen I (PG1), basal gastrinemia and basal acid output (BAO) and maximal acid output (MAO) were measured. Compared to controls, the mean levels of PG1 and gastrin were significantly higher in the patients; 12 children (80%) had high levels of PG1 and the remaining 3 (20%) had normal levels. The blood group O was the most prevalent: 64% of the cases.