The PCR was used to detect the presence of Helicobacter pylori in subgingival plaque samples from patients with adult periodontitis. Primers based upon the 16S ribosomal RNA (rRNA) gene sequence of H. pylori were used in a single round of PCR to amplify a 295-bp DNA fragment and the identity of the amplified products was confirmed by Southern blot hybridisation to a digoxigenin-labelled H. pylori probe. Further confirmation of product identity was obtained by DNA sequencing of a proportion of the amplified products. The assay was demonstrated to be specific for H. pylori with a lower limit of detection of 100 fg of bacterial genomic DNA. In all, 73 samples from 29 patients were analysed, of which 24 (33%) were H. pylori-positive by PCR; the proportion of patients carrying H. pylori in at least one sampled site was 38% (11 of 29). This is the first study to demonstrate the presence of H. pylori in the subgingival plaque of patients with adult periodontitis and indicates that, in this patient group at least, subgingival plaque may be a reservoir for H. pylori infection.