Objective: The purpose of this study was to evaluate the bactericidal effectiveness of using a fiber-less Er:YAG laser in the first stage of therapy for moderate chronic periodontitis and to compare it with conventional treatment. Method and Materials: Two quadrants from 20 patients with moderate chronic periodontitis were treated with Gracey curettes (control), and the contralateral two quadrants in each patient were treated using an Er:YAG laser with total power of 1.5W (test). Subgingival plaque samples from the four deepest pockets in each quadrant were taken immediately before and 1 month after treatment, and the presence of nine marker bacteria was studied using real-time polymerase chain reaction technology. Results: A significant reduction of total pathogens and bacteria from the red complex was observed 1 month after treatment with both procedures. The results were more significant for the test group (P = .003) than for the control group (P = .005). Qualitative analysis of sites that had a therapeutically significant number and proportion of marker bacteria also showed significant reduction after treatment. Conclusion: The results of the present study suggest that the Er:YAG laser possesses comparable with conventional treatment bactericidal effectiveness against periodontal pathogens in vivo in the initial treatment of moderate chronic periodontitis. More evaluations should be performed to prove these results for a longterm successful clinical outcome. Schlagwörter: antibacterial, curette, Er:YAG laser, periodontal disease