BACKGROUND AND AIMS: Approximately 60% of Helicobacter pylori (H. pylori) isolated in the Western world possess the cytotoxin-associated gene A (cagA), a known virulence factor. However, the frequencies of cagA positive H. pylori have been different geographically or racially. This study was performed to investigate the positive rates of cagA gene in Korean patients with chronic gastritis, gastric ulcer, duodenal ulcer or gastric eancer, and to compare the degree of gastritis between cagA gene positive and negative cases. METHODS: From 110 patients (31 patients with chronic gastritis, 32 patients with gastric ulcer, 36 patients with duodenal ulcer, and 11 patients with gastric cancer), four biopsies were obtained; one (angle) for CLO test, two (antrum and carpus) for histopathologic examination and one (antrum) for PCR. The cagA and ureC gene were assayed by PCR. The degrees of inflammation and activity of gastritis were graded by the Sydney system. RESULTS: Ninety four among 110 cases (85.5%) were positive in CLO test or histopathology. Among seventy two cases which were positive in CLO test and ureC assay, 64 cases (88.9%) were positive for cagA PCR. The positive rates of cagA PCR in patients with chronic gastritis, gastric ulcer, duodenal ulcer and gastric cancer were 89.5%, 83.3%, 93.1%, 83.3%, respectively. The degrees of inflammation and activity were significantly higher in H. pylori positive cases than in negative cases for both antrum and body biopsies (p<0.05). On the other hand, there was no difference between cagA gene postive and negative cases. The H. pylori density of antrum was significantly higher in cagA gene positive cases than in negative cases (p<0.05). Canelusions: The prevalence of cagA gene positive cases was high and not different among diseases, and there was no difference on the degree of gastritis between cagA gene positive and negative cases. Therefore, we think that the role of eagA as a virulence factor is weak in Korea.