We examined whether 13C-urea breath test with powdered 13C-Urea is affected with urease activity in the oral cavity and oropharynx in H. pylori negative cases. We also examined whether the degree of gargling affects the results of 13C-UBT. The results demonstrated that the urease activity not only in the oral cavity, but in the oropharynx may influence the UBT value. Therefore, it is possible that the UBT value may be affected with relic urease activity in the oropharynx if only the oral cavity is gargled. In conclusion, Gargling of the oropharynx in addition to the oral cavity is important to obtain true negative diagnosis with 13C-UBT in patients without Helicobacter pylori infection.