Infection with Helicobacter pylori (H. pylori) increases stomach cancer risk. Helicobacter pylori strains with the cag pathogenicity island (PAI) induce more severe inflammation in the gastric epithelium and are more strongly associated with stomach cancer risk than strains lacking the PAI. We examined whether the prevalence of somatic p53 mutation in gastric adenocarcinoma differed between subjects with and without infection with CagA(+) (a marker for the PAI) H. pylori strains. DNA from 105 microdissected tumor specimens was analyzed for mutation in exons 5-8 of the p53 gene by polymerase chain reaction-based single-strand conformation polymorphism followed by direct DNA sequencing. Enzyme-linked immunosorbent assays for IgG antibodies against H. pylori and CagA were performed on sera collected 2-31 years prior to cancer diagnosis. Tumors from CagA(+) subjects were significantly more likely to have p53 mutations than tumors from CagA(-) subjects (including H. pylori- and H. pylori(+)/CagA(-)): odds ratio = 3.72; 95% confidence interval, 1.06-13.07 after adjustment for histologic type and anatomic subsite of tumor and age at diagnosis and sex of subjects. Mutations were predominantly insertions and deletions (43%) as well as transition mutations at CpG dinucleotides (33%). The data suggest that CagA(+) H. pylori infection, when compared with CagA(-) infection or the absence of H. pylori infection, is associated with a higher prevalence of p53 mutation in gastric adenocarcinoma.