Objectives: To assess the effects of an experimental 0.454% stannous fluoride (SnF2) dentifrice on the oral sulcular microbiota in patients with various stages of oral diseases using checkerboard DNA-DNA hybridization. Material and Methods: In the present one-month, single center, single product, prospective cohort trial, 37 adults (mean age 37.6) were assigned to one of four oral health condition cohorts with seven to 10 subjects each: 1. mild gingivitis, 2. marked generalized gingivitis to moderate periodontitis, 3. caries-prone and 4. treated moderate to advanced chronic periodontitis in supportive periodontal care. All four groups were asked to use the test dentifrice and a power toothbrush twice a day for one minute during a four-week test period. Before and after the trial period, Plaque Indices (PlI, Silness and Löe, 1964) and Gingival Indices (GI, Löe and Silness, 1963) were recorded. Subgingival plaque samples were collected from all patients at Baseline, as well as after two and four weeks. These samples were analyzed for content of 40 bacterial species using checkerboard DNA-DNA hybridization. Results: As a result of the only one minute brushing with the stannous fluoride dentifrice, the mean PlI at Baseline was significantly lower (p 0.05) from the mean PlI at four weeks. No statistically significant differences were found between premolar and molar mean values. Moreover, no statistically significant differences were found between the mean GI at Baseline and at four weeks. The microbiological analysis showed that at baseline subjects in groups 2 and 4 had significantly higher bacterial loads of bacteria than groups 1, and 3 (i.e. A.actinomyctemcomitans P.gingivalis, T.forsythia, and T.denticola. Over the study period, the total bacterial load did not change in groups 2, 3 and 4. In groups 1 and 3, however, an increase in the loads of Streptococci spp. were noticed (p 0.05) including S.mitis, S.intermedius, and S.sanguis (p 0.01) suggesting an increase in the presence of early colonizing and health associated bacteria. Conclusion: One minute brushing with a 0.454% stannous fluoride dentifrice did - after four weeks - not affect the subgingival microbial profiles in patients with moderate periodontitis and treated moderate to advanced periodontitis. However, the sulcular microbial profiles of mild gingivitis and caries-prone patients were affected, indicating a shift towards a gingival health associated microbiota in the sulcular region of patients not affected by attachment loss. Running Head: Effect of stannous fluoride on sulcular microbiota Keywords: biofilm, dentifrice, DNA-DNA hybridization, gingivitis, periodontitis, stannous fluoride