OBJECTIVE To evaluate the effect of mechanical periodontal treatment combined with tetracycline on periodontal attachment, distances of cementum-enamel junction (CEJ) to the bone defect bottom and to the alveolar crest, and avidity of serum IgG against Porphyromonas gingivalis (Pg) lipopolysaccharide (LPS) in patients with aggressive periodontitis. METHODS Twenty-six patients with aggressive periodontitis were divided equally into two groups to receive mechanical periodontal treatment combined with tetracycline or exclusive mechanical periodontal treatment, with another 20 periodontally healthy subjects serving as the control group. In the two patient groups, routine clinical examination of the subjects was performed to record periodontal attachment level and distances of CEJ to the bottom of the bone defects and to the alveolar crest before and 3 and 6 months after the treatment. The avidity of serum IgG against Pg LPS was measured by ELISA with thiocyanate elution before and after periodontal treatment. RESULTS Compared with patients with exclusive mechanical periodontal treatment, patients with combined treatments showed significant improvement in periodontal attachment level and the distance of CEJ to the bone defect bottom after the treatment (P<0.01 and P<0.05, respectively) with also significantly reduced avidity of serum IgG against Pg LPS (P<0.01). CONCLUSIONS Mechanical periodontal treatment combined with tetracycline produces favorable effects in patients with aggressive periodontitis.