AIM: To evaluate the association of pre-treatment 13C-urea breath test (UBT) results with H pyloridensity and efficacy of eradication therapy in patients with active duodenal ulcers.METHODS: One hundred and seventeen consecutive outpatients with active duodenal ulcer and H pyloriinfection were recruited. H pylori density was histologically graded according to the Sydney system. Each patient received lansoprazole (30 mg b.i.d.), clarithromycin (500 mg b.i.d.) and amoxicillin (1 g b.i.d.) for 1 week. According to pre-treatment UBT values, patients were allocated into low (＜16%o),intermediate (16-35%o), and high (＞35%o) UBT groups.RESULTS: A significant correlation was found between pre-treatment UBT results andHpyloridensity (P＜0.001).H pylorieradication rates were 94.9%, 94.4% and 81.6%in the low, intermediate and high UBT groups, respectively (per protocol analysis, P=0.11). When patients were assigned into two groups (UBT results ≤35%o and ＞35%o),the eradication rates were 94.7% and 81.6%, respectively (P=0.04).CONCLUSION: The intragastric bacterial load of H pylori can be evaluated by UBT, and high pre-treatment UBT results can predict an adverse outcome of eradication therapy.