Antacids are widely used in the management of duodenal ulcer but the optimum dose of antacid required for ulcer healing has not been determined. We therefore studied 107 patients with endoscopically diagnosed duodenal ulcer who were allotted at random to one of the following treatment groups; placebo (group P) and antacid (groups A, B and C). A liquid antacid (Aludrox MH, Wyeth) with neutralising capacity of 2.3 mmol HCl/ml was administered in graded doses of 7.5 ml (Group A), 15 ml (Group B), and 30 ml (Group C), one hour and three hours after each meal, six times a day for four weeks. Patients in group P received 15 ml liquid placebo in a similar fashion. Complete symptomatic relief was obtained in 33% of patients in the placebo group, 54% in antacid group A, 89% in group B, and 92% in group C. Endoscopic assessment at the end of four weeks of treatment gave an ulcer healing rate of 29% in the placebo group, 46% in group A (103.5 mmol antacid/day), 85% in group B (207 mmol/day), and 88% in group C (414 mmol/day). There was no significant difference in the healing rates and pain relief between placebo and antacid group A, while both groups B and C had significantly higher ulcer healing rates and pain relief compared with placebo (p less than 0.001) and antacid group A (p less than 0.01). Drug related unwanted effects were recorded only in group C - 28% of patients suffered from diarrhoea. It is concluded that the optimum antacid requirements for the treatment of duodenal ulcer is 90 ml (acid neutralising capacity, 207 mmol HCl) per day.