SUMMARY: This study was designed to study the in vivo prevalence and the heterogeneity of H, pylori in patients with gastroduodenal diseases in central Taiwan. H. pylori infection was detected in 74.1% (575/776) of the symptomatic population studied. The prevalence of H. pylori infection increased from 11.1% in those between the ages of one to 20, to 82.9% in those between the ages of 41 and 50, and to 84% in those between the ages of 51 and 60. There was no significant difference in the prevalence of H. pylori infection between men and women. Among different blood types, the prevalence and relative risk of H. pylori infection was significantly higher in blood group O patients (90.3%) than in blood group A (41%), blood group B (27.4%), or blood group AB (62%) patients. Metronidazole resistance was found in 6.7% of the primary isolates. The prevalence of metronidazole-resistant H. pylori strains was higher in women (7.69%) than in men (6.25%), but this difference was not significant. A total of 88% of H. pylori strains were cagA-positive. CagA gene-positive strains were present in 90.1% of duodenal ulcers, 90% of duodenal ulcers combined with gastric ulcer, 85.8% of gastric ulcers, and 69.2% of gastritis patients, and was significantly higher in peptic ulcer disease groups than in the gastritis group. In conclusion, there was a low incidence (6.7%) of metronidazole-resistant H. pylori strains and a high prevalence (88%) of H. pylori cagA -positive strains in central 13