Forty patients with endoscopically proven duodenal ulceration have cooperated in a clinical trial to compare the ulcer healing effect of colloidal bismuth subcitrate (CBS) at standard dosage administered either twice or four times a day. No statistically significant difference was found to exist between ulcer healing in the two groups after 4 weeks (70% and 95%, respectively), and it is concluded that twice daily CBS is an effective treatment for duodenal ulcer with clear advantages as regards patient cooperation.