In a randomized, endoscopically controlled trial, the effects of colloidal bismuth subcitrate (CBS) tablets on the healing of chronic duodenal ulcers were compared with those of ranitidine (R). 38 patients were originally allocated to the CBS group and 37 to ranitidine. There were 5 dropouts in each group. Of 33 patients given CBS, 25 (75%) showed complete ulcer healing by 4 weeks and 30 (91%) by 8 weeks. Of the 32 patients given ranitidine, 28 (87%) showed complete ulcer healing by 4 weeks and 30 (94%) by 8 weeks. There was no significant difference between these results. Symptom resolution was similar in both groups and no significant adverse effects were noted.