Thirty-seven outpatients with benign gastric ulcers proved by endoscopy were randomized and treated with tripotassium dicitrato-bismuthate (TDB) and a placebo for four weeks. They were assessed on the basis of symptoms and endoscopy examination. Ulcer healing observed by endoscopy was significantly better in the TDB treated group (18 of 20) than in the placebo treated group (six of 17). There was no statistical difference in symptomatic improvement. Nineteen patients with duodenal ulcers proved by endoscopy were randomized in a double-blind trial and given TDB and a placebo for four weeks. Ulcer healing in the TDB treated group (eight of nine) was significantly better than in the placebo treated group (two of 10). No significant difference was found in the symptomatic improvement of both groups. It is concluded that TDB is an effective agent for the treatment of chronic gastric and duodenal ulcers.