Intraluminal pH was measured simultaneously in the human stomach and proximal duodenum with six small glass electrodes tied together at 1.5-cm intervals. Twenty-four healthy control subjects and 44 patients with duodenal ulcer disease were studied under fasting conditions and for 3 h after a standard liquid meal. Mean and median hydrogen ion activity, percentage of time with pH below 2 and 3, and the frequency of pH fluctuations were calculated from digital pH data sampled at a frequency of once per second from each electrode. None of these measurements of acidity differed significantly between the two groups or between subgroups of normosecretor controls and hypersecretor ulcer patients. At the time of pH study 15 of the patients had endoscopically verified active ulcer disease and 13 patients were without disease activity. Gastric as well as duodenal bulb acidity was the same in these two subgroups. We conclude that even though duodenal ulcer patients deliver more acid into the duodenum, this does not cause increased luminal acid aggression.