A single calibrated examiner, blinded to smoking status, assessed periodontal clinical outcomes (recession, pocket depth, clinical attachment level, plaque index and bleeding on probing). A structured questionnaire was applied in order to collect demographic and behavioral information. Expired carbon monoxide concentration was measured with a monoximeter, in order to validate smoking status. A pooled subgingival plaque sample was collected from the deepest periodontal pocket from each participant. The presence and quantification of Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola were determined using real time - PCR (RT-PCR).