The aim of this prospective interventional study is to verify the efficacy of smoking cessation on clinical and microbiological outcomes of non-surgical periodontal therapy of chronic periodontitis patients. Smokers willing to quit received periodontal treatment and concurrent smoking cessation therapy. Periodontal maintenance was performed every 3 months. A single calibrated examiner, blinded to smoking status, assessed periodontal clinical outcomes and applied a structured questionnaire in order to collect demographic and behavioural information. Further, expired carbon monoxide concentration were measured with a monoximeter. 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 (RT-PCR).