Gastric acid secretion before and after stimulation with pentagastrin assessed in 11 patients with Crohn's disease. Age and sex-matched patients with ulcerative colitis and healthy controls were used for comparison. Basal acid output was found to be significantly elevated in patients with Crohn's disease who had had surgery (P less than 0.01). This elevation was related to ileal resection rather than ileal disease. There was no difference in the maximal or peak acid outputs between any of the groups.