BACKGROUND: The aim of this study was to investigate the prevalence of Helicobacter pylori in the saliva of infected patients and the relation between H. pylori in the saliva and the severity of gastric infection. METHODS: Active gastric infection was determined by the 13C-urea breath test. Bacteria in saliva were detected by the nested polymerase chain reaction, using primer sets EHC-U/EHC-L and ET-5U/ET-5L. RESULTS: The PCR assay was able to detect as few as 5 H. pylori /mL. A total of 82 (71.9%) out of 114 patients with gastroduodenal diseases were positive by 13C-urea breath tests. Among these 82 patients, 21 (25.6%) were PCR positive in their saliva. H. pylori was present in the saliva of patients with highly active gastric infections (> 50 delta perrmille). No H. pylori was detected in saliva of patients with no active gastric infections. CONCLUSIONS: In patients with highly active gastric infections, H. pylori may be transmitted via saliva. The PCR assay of H. pylori in saliva is not useful for detecting gastric infection but may be a useful tool for the screening of highly infectious patients.