Background/Aims: 13C-urea breath test (UBT) is an useful diagnostic test for Helicobacter pylori (H. pylori) infection. This study was aimed at clarifying the relationship between the UBT and gastric histological findings. Methods: Forty-nine patients with H. pylori infection identified by UBT and histological findings were selected. For biopsy specimens obtained endoscopically from gastric antrum and body, the number of H. pylori, gastritis activity, atrophy and intestinal metaplasia were inverstigated. Their gastritis activities were graded according to the updated Sydney system. UBT results were expressed delta over baseline at 30 min (DOB30). Results: The grades of gastritis activity were classified histologically into scores of 0, 1, 2, and 3. The DOB30 values were positively correlated with the grades of gastritis activity (r=0.35, p<0.05). However, there was a negative correlation between the DOB30 values and the grades of gastric mucosal atrophy (r=-0.39, p<0.05). In concerning with existence of intestinal metaplasia, the DOB30 values were 29.0±25.3‰, 55.1±38.9‰, 42.1±26.8‰ in none, focal, broad existence of intestinal metaplasia, respectively (r=0.01, p>0.05). A significant positive correlation was observed between the DOB30 values and intragastric H. pylori load (r=0.52, p<0.01). Conclusions: 13C-UBT can be used to evaluate intragastric H. pylori load, grades of gastritis activity and atrophy. (Korean J Gastroenterol 2001;37: 76-81)