Purpose: To determine the preventive effect of a periodontal dressing containing colophony, zinc oxide and magnesium oxide applied after scaling and root planing on clinical variables, subgingival bacteria and local immune response in patients with chronic periodontitis. Materials and Methods: In this randomised prospective clinical study, 28 volunteers with generalised moderate chronic periodontitis were treated with full-mouth scaling in a split-mouth design. In the test quadrants, the periodontal dressing was applied during the first three days. At baseline and after 6 and 12 weeks, probing pocket depth (PD), attachment level (AL) and bleeding on probing (BOP) were recorded, and subgingival plaque samples were taken for laboratory analysis. Results: In both groups, PD, AL and BOP were significantly reduced (p = 0.001). BOP was significantly lower in the control than the test group after 6 weeks (p = 0.046). Significantly reduced bacterial counts of Porphyromonas gingivalis were found in the control group after 12 weeks (p = 0.013). No differences were found for the microbiological results between the groups. After 12 weeks, interleukin (IL)-8 and matrix metalloproteinase (MMP)-8 were significantly higher in the test group (p = 0.023 and p = 0.003, respectively). Conclusion: The adjunctive application of a periodontal dressing had no additional preventive effect on clinical data 12 weeks after scaling and root planing. Keywords: bacteria, chronic periodontitis, cytokines, periodontal dressing, root planing