The primary aim of this study was to assess if the probiotic Lactobacillus reuteri could be effective in the treatment of gingivitis and further to evaluate the influence of the probiotic on plaque and the lactobacilli population in the saliva. A randomised, placebo-controlled, double blind study was performed over 2 weeks. Fifty-nine patients with moderate to severe gingivitis were included and given one of two different Lactobacillus reuteri formulations (LR-1 or LR-2) at a dose of 2 x 10(8) CFU per day, or a corresponding placebo. At baseline (day 0) gingival index and plaque index were measured on two surfaces and saliva for lactobacilli determination was collected. The patients were instructed how to brush and floss efficiently and study treatment was started.The patients returned on day 14 for final assessment of gingivitis and plaque and saliva was collected. 20 patients were randomised to LR-1, 21 to LR-2 and 18 to placebo. Gingival index fell significantly in all 3 groups (p < 0.0001). LR-1, but not LR-2 improved more than placebo (p < 0.0001). Plaque index fell significantly in LR-1 (p < 0.05) and in LR-2 (p < 0.01) between day o and day 14 but there was no significant change in the placebo group. At day 14, 65% of the patients in LR-1 were colonised with Lactobacillus reuteri and 95% in the LR-2 group. Lactobacillus reuteri was efficacious in reducing both gingivitis and plaque in patients with moderate to severe gingivitis.