This study determined the effect of brushing with 0.4% stannous fluoride (SnF2) or 0.22% sodium fluoride (NaF) on clinical and microbial parameters associated with gingivitis. The study included three groups of 281 subjects. Subjects in all three groups were instructed to brush twice daily with an ADA-accepted fluoride dentifrice, rinse their mouths with water, and subsequently brush with 0.4% SnF2, 0.22% NaF, or a fluoridefree placebo gel. More stain was detected in the SnF2 group than in the other two groups at all periods except at baseline. However, no differences were observed in gingivitis, bleeding, or mean proportions of microbial forms in the SnF2 or NaF groups when compared with the placebo group at 18 months. Results indicate that 0.4% SnF2, or 0.22% NaF is no more effective than a placebo in reducing gingivitis.