BACKGROUND/AIMS: Efficacy of triple therapy with omeprazole, amoxicillin, and clarithromycin (OAC) for Helicobacter pylori (H. pylori) varies among different geographic regions and patient populations. And, there is no full consensus on how long this treatment should be maintained. We assessed the efficacy of the OAC regimen according to the treatment duration (7, 10 or 14 days) using 13C-urea breath test (13C-UBT). METHODS: Two hundred fifty five H. pylori positive patients with peptic ulcer were randomly assigned to a OAC (omeprazole 20 mg b.d., amoxicillin 1.0 g b.d. and clarithromycin 500 mg b.d.) 7, 10 or 14 days regimen. Eradication of H. pylori was assessed by 13C-UBT 4 weeks after the completion of therapy. RESULTS: 140 male and 115 female (mean age, 51.3) patients were enrolled. The overall eradication rate of H. pylori in each group was significantly higher in OAC 14 days regimen than OAC 7 and 10 days regimen (91.9% in OAC 14 days, 74.4% in OAC 7 days, and 80.2% in OAC 10 days, respectively, p<0.05). CONCLUSIONS: The overall eradication rate of H. pylori was highest in OAC 14 days regimen in our study. We have found significant differences in eradication rates with previous reports, using CLO and histology, in Korea. Thus, further studies focusing on the treatment period may be warranted.