Porphyromonas gingivalis (P. gingivalis) is considered to be a pathogenic factor in adult or rapidly progressive periodontitis. The purpose of this study was to evaluate the distribution of P. gingivalis in the dentition of adult periodontitis patients using a nonradioactive DNA probe, and to compare the presence of P. gingivalis with clinical parameters. Twelve adult periodontitis patients were examined. Subgingival plaque samples were taken from 4 sites of all the remaining teeth using a paper point. At the same time, probing depth and bleeding on probing (BOP) were also recorded. Plaque samples were investigated using a whole genomic DNA probe from P. gingivalis (ATCC 33277) modified with bisulfite. The detection, percentage and amounts of P. gingivalis present were statistically compared with probing depth and BOP in each patient. P. gingivalis was detected in all patients examined. The detection percentage was 35% of all sample sites. When the probing depth was over 4 mm or BOP was positive, the detection percentage of P. gingivalis significantly increased (P < 0.01). As more P. gingivalis was identified, the percentage of sites with deep probing depth or that were BOP positive increased significantly (P < 0.01). However, P. gingivalis was also detected in clinically healthy sites, and P. gingivalis negative sites with deep probing depth or that were BOP positive existed in the same patient. These results indicate that P. gingivalis play an important role, but is not the only microorganism responsible for adult periodontitis.