The genome of H. pylori is 1.68-1.73 Mb in size and contains a relatively low GC content (an average of 32.5 mol%). Physical and genetic maps of five H. pylori strains (NCTC 11637, NCTC 11638, NCTC 11639, UA 802 and UA 861) have been constructed and the complete genome sequence of strain 26695 has been determined. At least 50 genes, some of which play important roles in the physiology and pathogenicity of the bacterium, have been cloned. Marked genomic sequence variability has evolved from stain to strain demonstrated by random arrangement of 17 known genes on the chromosome and frequent mutations within individual genes. Based on such variability, sensitive and efficient molecular tying techniques such ribotyping, AR-PCR, PCR-RFLP, PCR-DNA sequencing and PFGE-RFLP have been developed and widely applied in both epidemiological and clinical studies of this pathogen. Subtypes of vacA (encoding a vacuolating cytotoxin) and the intermediate forms of a pathogenicity island (the cag region) have been identified in different H. pylori strains and these individual vacA subtypes are associated with specific clinical manifestations of H. pylori infection. Further studies on relationships between the genetic diversity and pathogenicity of H. pylori strains would lead to the development of novel and efficient therapeutic strategies for eradication of this microorganism.