The efficiency of the 13C-Urea Breath Test (13C-UBT) for the detection of Helicobacter pylori colonization in gastric mucosa was evaluated. The 13C-UBT was performed in five pediatric and six adult subjects who had had upper gastrointestinal endoscopy within 2 weeks. H. pylori colonization was confirmed in two pediatric and three adult subjects with peptic ulcer combined with antral gastritis, by histological examination of antral biopsy specimens. When an individual with H. pylori colonization ingested a solution containing 13C-urea, a significant amount of 13CO2 appeared in the respiratory CO2 within 10 min. The mean cumulative percentage dose of 13C recovered in the breath over 30 min in the cases with H. pylori colonization was significantly higher than that in those who were not colonized (4.91 vs 0.41, P < 0.001). In addition, the effect of antibiotic on the eradication of H. pylori from gastric mucosa was monitored by 13C-UBT in two cases. The values of cumulative percentage dose of 13C over 30 min fell to the same levels as those observed in H. pylori negative subjects after just 2 weeks treatment with amoxicillin; however, positive results were obtained again 1 month after the withdrawal of amoxicillin. In summary, 13C-UBT is a simple, reliable, non-invasive method in the diagnosis of gastric H. pylori colonization especially for pediatric patients.