CagA-positive H. pylori is reported to be associated with gastroduodenal disease in Western countries. To evaluate the relationship between CagA and disease, cloning of the entire cagA gene (3771 bp), insertion of a partial fragment (1272 bp) into an expression vector, purification of the recombinant protein, production of an antibody against the recombinant CagA protein through rabbits, and use of the recombinant CagA protein as an antigen, detection of the anti-CagA antibody by western blotting were all performed. Sera of 132 H. pylori-infected patients undergoing endoscopy were studied. Anti-CagA antibodies were detected in 90%, 87%, 90%, 94%, and 93% of patients with gastric ulcer (N = 34), duodenal ulcer (N = 27), chronic gastritis (N = 31), gastric cancer (N = 17), and normal mucosa (N = 15), respectively. High seropositivity of anti-CagA antibody even in individuals with normal mucosa indicated that CagA may not be a unique marker for disease by H. pylori infection in Japan.