We examined the biologic tumor behavior in Helicobacter pylori-seropositive patients with gastric adenocarcinoma. A total of 214 consecutive patients with pathologically confirmed adenocarcinoma of the stomach who underwent gastric resection were studied. The stored serum samples were tested for serum antibody to H. pylori by using a highly sensitive and specific IgG enzyme-linked immunosorbent assay. The difference in H. pylori-seropositive and seronegative patients with gastric adenocarcinoma was evaluated in terms of various clinicopathologic parameters. A multivariate logistic regression analysis was used to adjust for potential confounding variables. Antibodies to H.pylori were detected in 65.9% of patients with gastric adenocarcinoma. H.pylori-seropositive patients were younger than seronegative patients and had infiltrative tumor according to Ming's criteria. When adjusted for age, infiltrative tumor come out stronger. These findings suggest that H.pylori infection may be related to infiltrative type gastric adenocarcinoma; further study is necessary.