Periodontal disease is a complex infectious disease caused by bacteria in the oral mucosa, which results in gingival inflammation, breakdown of periodontal tissues, bone resorption, and finally tooth loss. Mechanical plaque control methods-scaling and root planing are effective methods to stop the progression of such periodontal disease. It was reported that subantimicrobial dose of doxycycline(SDD) regimen could improve clinical conditions of periodontal tissues without causing the overgrowth of opportunistic organisms that was a typical antibiotic side effect. Therefore pharmacological therapy, used in conjunction with mechanical therapy could be considered a useful treatment modality in the treatment of chronic periodontal disease. In this study, 30 patients diagnosed as moderate to advanced chronic periodontitis were divided into 2 groups. In this double-blind, placebo-controlled study, the patients were administered 20mg doxycycline capsule or placebo capsule b.i.d. for 4months, after scaling and root planing. Clinical parameters-bleeding on probing, pocket depth and clinical attachment level were compared and evaluated between these groups at periods of first visit, 1 month, 2 months, 3 months, 4 months. The results were as follows ; 1. In case of moderate periodontitis, pocket depth showed significant reduction after treatment in both the control & experiment groups, when compared with the baseline values(p<0.01), but in case of advanced periodontitis, only the experiment group showed significant reduction after treatment when compared with the baseline values(p<0.05). Statistically significant reduction in pocket depth was observed in the experiment group compared to the control group(p<0.05). 2. In case of moderate periodontitis, clinical attachment level showed significant reduction after treatment in both the control & experiment groups, when compared with the baseline values(p<0.01), but in case of advanced periodontitis, only the experiment group showed significant reduction after treatment when compared with the baseline values(p<0.05). Statistically significant reduction in clinical attachment level was observed in the experiment group compared to the control group(p<0.05). 3. Bleeding on probing improved after treatment in both the groups. In case of moderate periodontitis, the experiment group showed statistically significant reduction of bleeding on probing when compared with the control group at 1 and 4 months after treatment(p<0.05). In case of advanced periodontitis, treatment resulted in statistically significant reduction of bleeding on probing in both the groups(p<0.05). These results indicate that the use of subantimicrobial dose of doxycycline is a useful supplement to mechanical treatment for periodontal patients in ameliorating the clinical parameters such as periodontal pocket, attachment level, and bleeding on probing.