Porphyromonas gingivalis is considered to be the most important infectious agent causing several types of periodontal diseases, includinig adult periodontitis. This micro-organism has been frequently detected in patients with periodontitis, while it reportedly occurs at a lower frequency in periodontally healthy individuals without marked gingival inflammation. There may be diversities in virulence among the organisms harboured by individuals who are periodontally healthy and those with periodontitis. The aim of the study was to investigate the prevalence of virulent stains of Porphyromonas gingivalis in periodontal disease and health.