Colonization of Helicobacter pylori depends on virulence factors of the organism, host factors and environmental factors. The prevalence of H. pylori differs between developing and Western developed countries, suggesting that the socioeconomic status influences the acquisition of the infection. The prevalence of Japanese children has been almost equal to that of Western countries. Although a main mode of transmission remains unclear, childhood is critical for acquisition of the infection. H. pylori causes gastritis (especially nodular gastritis) and duodenal ulcers in children but rarely gastric ulcers. No pediatric cases with H. pylori-associated gastric cancer have been reported. However, precancerous lesions including mucosal atrophy can develop in the infected children. Further studies in children are needed on strategy for H. pylori infection including prophylaxis of gastric cancer.