Early studies have suggested that omeprazole may facilitate the eradication of Helicobacter pylori. Sixty patients with duodenal ulcer and H. pylori colonization were randomly assigned to receive either omeprazole monotherapy (n = 30) or combination therapy with omeprazole and amoxycillin (n = 30) for a total duration of 6 weeks. Four patients receiving monotherapy and three receiving combination therapy had to be withdrawn from the study. All (100%) duodenal ulcers healed in patients receiving combination therapy, and 25 out of 26 (96%) healed in the group receiving monotherapy. H. pylori was eradicated in 22 out of 27 (82%) patients receiving combination therapy; only two ulcer relapses (9%) occurred within 18 months in these 22 patients. Of the five patients who remained H. pylori-positive after combination therapy, two relapsed during the 18-month follow-up. In the monotherapy group, all patients remained H. pylori-positive after treatment, and duodenal ulcer relapsed in 16 out of 25 (64%) patients within the median follow-up of 18 months. Adverse events were not reported in the group treated with combination therapy; one patient receiving monotherapy reported severe headache. These results lend further support to existing data that H. pylori eradication prevents duodenal ulcer relapse and show that combination therapy with omeprazole and amoxycillin is effective and well tolerated.