Pyloric function after Ramstedt's pyloromyotomy was assessed in seven patients aged five to eleven years and compared to that in sixteen normal children. Gastric emptying (T 1/2) of liquid, as measured by the double sampling test, was faster in patients than in normal children. Duodenal reflux was calculated from the sodium content in gastric aspirates. At rest, it was greater in patients than in normal children. In the poststimulatory state, there was no difference. Gastric acid secretion was similar in both groups. Rapid gastric emptying might explain the high incidence of peptic ulcer reported in several series of long-term follow-up patients. Increased duodenal reflux at rest might account for a similar increased incidence in gastritis and dyspepsia.