Helicobacter pylori, a Gram-negative bacterium associated with a spectrum of benign and malignant gastric conditions, is one of the most genetically variable pathogens. Variations in a few genes have been associated with risk of inflammation and carcinogenesis, but to date no systematic study has evaluated the entire bacterial genome. We propose a multicountry study to establish and analyze an informative international collection of H. pylori clinical isolates from residents of defined geographic areas. Molecular characterization will be conducted at the NCI/FNLCR. Our hypothesis is that genetic and epigenetic variations in H. pylori strains among chronically infected individuals may affect risk of progression to gastric cancer. Findings from this study may help to characterize the differential virulence among H. pylori isolates and suggest predictive biomarkers for early diagnosis of cancer. The bacterial isolates and databases from this study will also provide a foundation for further elucidation of H. pylori pathogenesis.