Statement of problem: Although bacterial plaque is the primary ethiologic factor in the initiation of chronic periodontitis, there is some predisposing factors such as smoking which may aggravates disease process and its roll appears to be considered more important recently.Purpose: The purpose of the present study was the evaluation of the effect of the quantity of cigarette smoking on periodontal tissue response following phase I therapy.Materials and method: Thirty patients (26 male and 4 female) with mean age of 48±11 years participated in this interventional and cross-sectional study. All patients suffered from generalized moderate chronic periodontitis. The patients were divided into three equal groups of 10 (heavy smokers, light smokers and non smokers), according to the number of cigarette consumption. The smoker groups had a history of cigarette consumption at least for the past two years. Clinical probing depth, clinical attachment level and bleeding on probing were evaluated for the patients at baseline and eight weeks following completion of phase I therapy. The results were analyzed using Wilcoxon rank and Kruskal- Wallis tests.Results: No statistically significant differences were found in the evaluated variables between the groups except for BOP at baseline. All parameters decreased significantly eight weeks after completion of phase I therapy, but in comparison between three groups, only differences of CPD reduction and CAL gained between heavy and non-smokers were statistically significantConclusion: The present study indicated that heavy cigarette smoking have a negative effect on periodontal tissue response to phase I therapy. This effect depends on the number of daily smoked cigarettes and is increased with more than 20 cigarettes smoking per day.Key words: Response to periodontal treatment, Non surgical periodontal therapy, Smoking