Purpose: Heat-killed Lactobacillus plantarum L-137 (HK L-137) has been shown to activate innate and acquired immunity in humans. The aim of this randomised, double-blind, placebo-controlled clinical trial was to examine the effects of the oral administration of HK L-137 on the outcome of periodontal therapy. Materials and Methods: Thirty-nine patients undergoing supportive periodontal therapy (SPT) were randomly assigned to receive a capsule containing 10 mg of HK L-137 or a placebo capsule daily for 12 weeks. Nineteen patients in the experimental group and 17 patients in the control group were followed-up. Clinical parameters, including plaque index (PI), gingival index (GI), bleeding on probing (BOP), and probing depth (PD) were scored at baseline and weeks 4, 8 and 12 prior to prophylaxis in conjunction with regular SPT visits. Results: BOP and the number of teeth or sites with PD ≥ 4 mm were significantly reduced in both groups by a successive SPT programme, while there was significantly greater PD reduction (p 0.05) at teeth with site(s) with PD ≥ 4 mm at baseline in the experimental group than in the control group at week 12. Conclusion: These clinical findings suggest that daily HK L-137 intake can decrease the depth of periodontal pockets in patients undergoing supportive periodontal therapy. Keywords: dental plaque, gingivitis, Lactobacillus plantarum, probing depth, randomised clinical trial