AIMS--To determine the prevalence of Helicobacter pylori colonisation in the mouths of patients with H pylori gastritis. METHODS--A nested polymerase chain reaction test for the 16S ribosomal RNA gene of H pylori was used on saliva, dental plaque, gastric juice and gastric biopsy specimens from patients attending a dyspepsia clinic. RESULTS--Thirteen patients had histologically confirmed Helicobacter associated gastritis. Twelve of these had positive gastric aspirates by PCR. Five had at least one positive oral specimen. Eight patients with normal gastric biopsy specimens had no PCR positive oral specimens or gastric aspirates. All, however, had PCR positive gastric biopsy specimens. In an attempt to determine the origin of these positive results in normal patients, it was shown that biopsy forceps could contaminate specimens with DNA from previous patients. CONCLUSION--The demonstration of the organism in the mouths of a substantial proportion of dyspeptic patients has major implications for the spread of H pylori and identifies a potential source for reinfection following eradication of the organism from the stomach.