ABSTRACTInfection withHelicobacter pyloriis a major risk factor for development of gastric disease, including gastric cancer. Patients infected withH. pyloristrains that express CagA are at even greater risk of gastric carcinoma. Given the importance of CagA, this report describes a new molecular mechanism by which thecagAcopy number dynamically expands and contracts inH. pylori. Analysis of strain PMSS1 revealed a heterogeneous population in terms of numbers ofcagAcopies; strains carried from zero to four copies ofcagAthat were arranged as direct repeats within the chromosome. Each of the multiple copies ofcagAwas expressed and encoded functional CagA; strains with morecagArepeats exhibited higher levels of CagA expression and increased levels of delivery and phosphorylation of CagA within host cells. This concomitantly resulted in more virulent phenotypes as measured by cell elongation and interleukin-8 (IL-8) induction. Sequence analysis of the repeat region revealed threecagAhomologous areas (CHAs) within thecagArepeats. Of these, CHA-ud flanked each of thecagAcopies and is likely important for the dynamic variation ofcagAcopy numbers. Analysis of a large panel of clinical isolates showed that 7.5% ofH. pyloristrains isolated in the United States harbored multiplecagArepeats, while none of the tested Korean isolates carried more than one copy ofcagA. Finally,H. pyloristrains carrying multiplecagAcopies were differentially associated with gastric disease. Thus, the dynamic expansion and contraction ofcagAcopy numbers may serve as a novel mechanism by whichH. pylorimodulates gastric disease development.IMPORTANCESeverity ofH. pylori-associated disease is directly associated with carriage of the CagA toxin. Though the sequences of the CagA protein can differ across strains, previous analyses showed that virtually allH. pyloristrains carry one or no copies ofcagA. This study showed thatH. pylorican carry multiple tandem copies ofcagAthat can change dynamically. Isolates harboring morecagAcopies produced more CagA, thus enhancing toxicity to host cells. Analysis of 314H. pyloriclinical strains isolated from patients in South Korea and the United States showed that 7.5% of clinical strains in the United States carried multiplecagAcopies whereas none of the South Korean strains did. This study demonstrated a novel molecular mechanism by whichH. pyloridynamically modulatescagAcopy number, which affects CagA expression and activity and may impact downstream development of gastric disease.