The circadian pattern of intragastric acidity was assessed in 19 healthy subjects and 37 patients with active, endoscopically proven duodenal ulcer using 24-hr continuous intraluminal pH-metry. The median pH 24-hr profiles showed that ulcer patients had lower postprandial pH elevations and a smaller decline in acidity during the early morning hours when compared with controls. The after-lunch and -dinner area under the curve and maximum pH values were significantly higher in controls compared to ulcer patients. In the nighttime, the median pH values in controls were significantly higher during 9 PM to 12 PM (P = 0.02), 12 PM to 4 AM (P = 0.01), and 4 AM to 8 AM (P = 0.0008) compared to the ulcer patients. We conclude that the 24-hr acidity is higher in ulcer patients compared to healthy subjects and that the differences are particularly evident in the postprandial and nocturnal periods.