The purpose of this 3-week, double-blind study was to determine the effect of a stannous fluoride-containing mouthrinse on existing and developing dental plaque. A total of 55 subjects (mean age = 31.42 yrs.) received a professional prophylaxis in randomly assigned contralateral quadrants and were then stratified into two balanced groups based on screening plaque scores: one group (27 subjects) used the placebo rinse, the second group (28 subjects) used the test mouthrinse (0.63% diluted to 0.1% stannous fluoride). Plaque index (PI) and stain index (SI) were scored at baseline, week 1 and week 3. Gingival inflammation (GI) was monitored as a measure of product irritancy potential. The PI for the stannous fluoride rinse was significantly lower than the placebo, (p < 0.0001), for both prophied and unprophied sites with an average reduction of 29% at week 1 and 28% at week 3. There was no statistically significant difference between the presence or absence of prophylaxis. Plaque indexes for both stannous fluoride and placebo showed significant reduction (p < 0.0001) compared to baseline in all sites. Differences in staining potential between stannous fluoride and placebo were not significant (p > 0.05) at any time during the study. The stain index for both stannous fluoride and placebo showed a non-significant increase from baseline in the prophied and unprophied sites. No irritancy was noted, although a trend towards lower GI scores was observed at 3 weeks for the stannous fluoride group.(ABSTRACT TRUNCATED AT 250 WORDS)