A nested polymerase chain reaction (N-PCR) for the specific detection of Helicobacter pylori (H. pylori) was developed with two primer pairs derived from the urease gene A of H. pylori. The N-PCR was used to detect all 21 H. pylori strains, including 20 clinical isolates and 1 reference strain (NCTC 14126), but failed to detect 12 other bacterial species. It was shown that the PCR assay is 100% specific. Tenfold serial dilution experiments revealed that N-PCR could detect as little as 0.1 fg DNA. Samples of dental plaque and stomach biopsies from 29 patients were collected. Gastric specimens were tested by N-PCR, and the results were compared with those of culture, urease test and histological examination (reference standard). H. pylori sequences were detected by PCR showing that 21 were positive. The dental plaque samples from 8 of the 21 patients who tasted positive of gastric biopsies were positive by N-PCR. However, none of 8 dental plaque samples from the patients whose gastric samples were negative showed amplification. Dual therapy was given to two patients who were H. pylori positive in gastric mucosa and dental plaque. One month after treatment, H. pylori was eradicated from the gastric mucosa, but persisted in dental plaque.