Background: With the poor oral hygiene exhibited by patients with Down syndrome, the potential for a robust flora arises causing Periodontitis. Along with periodontal disease, the patient population has other medical conditions that limit treatment.  This study was conducted to evaluate the antibacterial effectiveness of an 870-nm diode laser on periodontitis in patients with Down syndrome.                               Subjects & methods :Thirty five patients with Down syndrome suffering from periodontitis were included in this study and divided into two groups, Group I (25 patients) laser was applied to one half of the mouth and the other half was considered as a control & Group II included (10 patients) basic periodontal treatment was applied to evaluate the systemic effect of laser. Quantification of A. actinomycetemconcomitans, P. Gingivalis and Capnocytophaga were carried out using real-time PCR technique at baseline and at 2 weeks, at 6 weeks and at 12 weeks post periodontal and laser treatment.   Results:  A. Actinmycetcomitans and P. gingivalis were the most common organisms found in the studied patients and Capnocytophaga was the least common organisms. In group I the quantitative results of A. actinomycetemconcomitans, P. Gingivalis at 2 weeks were significantly lower in both sides of mouth than that of baseline level.After 6 week, it was still significantly lower than that of the baseline in the right side of the mouth, but at 12 weeks no significant difference was observed in both sides. In group II, quantitative results of A. actinomycetemconcomitans, P. Gingivalis at 2 weeks were also significantly lower than that of baseline but not at 6 & at 12 weeks. As regards, Capnocytophaga there was no significant difference at baseline, at 2 weeks, at 6 weeks and at 12 weeks post periodontal and laser treatment in both groups I & II.   Conclusion, scaling and root planning plus laser therapy was better than scaling and root planning alone in the chosen patients and this antibacterial effect continues to be significant up to 6 weeks after therapy. . Based upon the results of the study, it was found that Laser therapy could be used for the treatment of periodontitis as a complementary therapy not to replace mechanical intervention.