Background/Aims: The eradication rates of regimens for Helicobacter pylori (H. pylori) infections are still around 85%. Lactobacilli have been demonstrated to have in vivo and in vitro inhibitory effects on H. pylori infection. We aimed to evaluate the inhibitory effect of Lactobacillus on H. pylori in human stomach. Methods: Forty H. pylori-infected volunteers (21-44 yr, F:M=9:31) were randomized into two groups which were treated with 100 mL of fermented milk containing lactobacilli (Lactobacillus acidophilus, Lactobacillus casei) q.I.d. or 100 mL of placebo q.I.d. for 4 weeks. Before and after treatment, they underwent 13C-urea breath tests (13C-UBTs) and endoscopies with biopsies for rapid urease test and histological evaluation. The compliance evaluated via daily checking questionnaire was considered acceptable if they took 90% of assigned treatment. Results: All patients were compliant and the H. pylori density of antrum tended to decrease in treatment group compared with placebo group (p=0.072). Δ13CO2 analyzed by Wilcoxon rank sum test showed a significant decrease in treatment group compared with placebo group (p=0.0064). Three cases in treatment group were noted for negative conversions of both rapid urease test and 13C-UBT. Conclusions: A fermented milk containing lactobacilli may have an inhibitory effect of H. pylori infection and can be an adjunct to the standard anti-H.pylori regimen. (Korean J Gastroenterol 2001;38:233-240)