The pathogenic role played by Helicobacter pylori in gastric mucosal defense was investigated in Japanese monkeys infected with H. pylori. Serum gastrin levels and ammonia concentrations in gastric juice were compared in H. pylori-infected (n = 6) and control (n = 7) groups. The gastritis score, the intracellular content of periodic acid-Schiff (PAS)-positive substance and hexosamine, and the bromodeoxyuridine (BrdU) labeling index in the gastric mucosa were compared in the two groups in the antrum and the corpus. The ammonia concentration in the gastric juice was significantly higher in the infected group (P < 0.01). The gastritis scores were significantly higher in the antrum and corpus in the infected group (P < 0.01, and P < 0.05, respectively). The content of PAS-positive substance and hexosamine was significantly decreased in the antrum of the infected group compared with that in the controls (P < 0.01, and P < 0.05, respectively), but there was no significant difference between the two groups in the corpus. The BrdU labeling indices were significantly higher in the antrum and corpus of the infected group (P < 0.01, and P < 0.01, respectively). Colonization by H. pylori injures the gastric mucosa by depressing the gastric mucosal defense factors, and, consequently, the cell kinetics are accelerated.