OBJECTIVE The aim of this two-year community demonstration trial was to study the caries inhibitory effects of semiannual applications of a fluoride varnish in preschool children. METHODS Twenty-four public dental health clinics in the county of Halland, Sweden, with 5,137 preschool children, 4 and 5 years of age, were matched and equally allocated to a fluoride varnish group (n = 2,535) and a reference group (n = 2,602). The children in the fluoride varnish group were treated every six months with topical applications of a silane fluoride varnish, Fluor Protector (0.1% F), while no fluoride varnish was used in the reference group. Both groups received a basic preventive program at annual checkups consisting of dietary counseling and instructions to parents to brush their children's teeth at least once daily with fluoridated dentifrice. Caries data were collected by clinical examinations at baseline and after one and two years. RESULTS Caries prevalence at baseline did not differ significantly between the groups. After two years, the mean caries incidence was low and no statistical difference was found in the total number of carious and filled surfaces (dfs) between the two groups. However, the incidence of approximal lesions (dfsa) was significantly lower (P < .05) in the fluoride varnish group than the reference group. Children in the fluoride varnish group with dfs scores of 1-4 and > or = 5 at the start of the study exhibited a statistically significant (P < .05) reduction in approximal caries incidence of 19 percent and 25 percent, respectively, when compared with the reference group. CONCLUSIONS Preschool children 4 and 5 years of age with clinical caries who receive semiannual applications of a silane fluoride varnish containing 0.1 percent F experience a reduced incidence of approximal caries over two years.