The effectiveness of a stannous fluoride mouthrinse, when used once each school day, was investigated in a 3-year study. Effervescent stannous fluoride tablets of two concentrations were dissolved in 20 ml of water, giving solutions of 100 parts/10(6)F- and 200 parts/10(6)F- respectively. Approximately 1,200 children, with a mean age of 10 years, were divided on a random basis into three groups. Two of the groups rinsed with the two strengths of solution and the third group rinsed with a placebo. Examinations were carried out at the commencement of the study, and at yearly intervals thereafter. The final series was carried out a year after the rinsing procedures were terminated. There were significant reductions in the numbers of new caries in each of the two experimental groups as compared with the controls. The concentration of the solutions appeared to have little influence on the results. More dramatic reductions were noted in the teeth which erupted during the course of the study. A residual effect was demonstrated a year after the rinsing procedures were terminated.