<p><b>OBJECTIVE</b>To study the relationship of the types of Helicobacter pylori (H. pylori) strains with the classification and the severity of chronic gastro-duodenal diseases in children.</p><p><b>METHODS</b>One hundred and fifteen children with chronic upper gastrointestinal symptoms who were diagnosed as H. pylori infection by gastroscopy were enrolled in this study. H. pylori strains were serotyped by immunoblot technique. The gastric biopsy specimens of all patients were studied histologically.</p><p><b>RESULTS</b>Type I H. pylori strains were confirmed in 84 cases (73.0%), intermediate type strains in 21 cases (18.3%), and type II strains in 10 cases (8.7%). Type I H. pylori strains infection caused a moderate gastric mucosal inflammation in 83 cases and a severe inflammation in 1 case. Intermediate type H. pylori strains infection caused a moderate gastric mucosal inflammation in 21 cases. Type II H. pylori strains infection caused a mild gastric mucosal inflammation in 2 cases and a moderate inflammation in 8 cases. Different types of H. pylori strains resulted in different severity of gastric mucosal inflammation (x2=15.444, P < 0.01). The gastric mucosal inflammation due to type I H. pylori strains was the most severe, while the inflammation due to type II H. pylori strains was relatively mild. The incidence of nodulus lymphaticus of gastric mucosa due to type I, type II and intermediate type H. pylori strains infection was 76.2%, 47.6% and 40.0%, respectively (x2=10.171, P < 0.01). The classification of chronic gastro-duodenal diseases was not associated with the types of H. pylori strains.</p><p><b>CONCLUSIONS</b>Type I strains were the leading cause of H. pylori infection in children. All of types of H. pylori strains can cause pathohistologic changes of gastric mucosa. Type I H. pylori strains infection can result in the most severe gastric mucosal inflammation and the highest incidence of nodulus lymphaticus. The immunoblot serotyping of H.pylori strains may be useless for the classification of chronic upper gastrointestinal diseases but it is helpful for the evaluation of the severity of the diseases in children.</p>