Background/Aims: Helicobacter pylori is an important factor in the significant gastrointestinal diseases. It has been suggested that the cytotoxin associated gene (cagA) is a marker of a more virulent strain of H. pylori. We tried to evaluate the prevalence of cagA-bearing H. pylori in patients with non-ulcer dyspepsia (NUD), duodenal ulcer (DU), and gastric ulcer (GU). Methods: Thirty eight NUD, 42 DU, 33 GU patients were included in this study. The rapid urease (CLO) test and PCR assay for the detection of H. pylori (ureA PCR) and cagA+ H. pylori (cagA PCR) were performed in the gastric mucosal biopsy specimen. Results: H. pylori were identified in 27/38 (71.1%) NUD, 38/42 (90.1%) DU, and 16/33 (48.5%) GU by rapid urease test. The ureA primer set amplified the 109bp of urease gene in 28/38 (73.7%) NUD, 34/42 (81.0%) DU, and 19/33 (57.6%) GU patients. The positivity of cagA+ strain in H. pylori were 18/28 (64.3%) NUD, 21/34 (61.8%) DU, and 11/19 (57.9%) GU patients when cagA PCR were performed in the H. pylori positive gastric mucosae. Although the prevalence of H. pylori in DU patients was higher than NUD patients, the prevalence of cagA+ strain of H. pylori did not differ in both groups. Conclusions: The prevalence of cagA+ strains in H. pylori positive gastric mucosae did not differ in the NUD, DU, and GU patients. There may be other important factors in the H. pylori associated pathogenesis of peptic ulcer diseases. (Korean J Gastroenterol 1998;31:184 - 191)