Sound evidence on the effectiveness of fluoride varnishes (FV) to reduce caries incidence in preschool children is lacking. Objective: To assess whether the application of FV in preschool children at 6-month intervals decreases the incidence of caries and produces any adverse effects. Methods: A randomized, examiner- and patient-blind, placebo-controlled, parallel-group design, clinical trial, comprising 1- to 4-year-old children, 100 in each group (FV or placebo varnish, PV), was conducted in Rio de Janeiro, Brazil. Two trained pediatric dentists performed the clinical examinations (kappa = 0.85). Dental caries was recorded at the d₂ (cavitated enamel) and d₃ (dentine) levels using the International Caries Diagnosis and Assessment System. Results: At baseline, the mean age of the participants was 2.4 years (SD 0.9) and the mean d₃mfs was 0.8 (SD 1.9). Most of the children brushed their teeth with fluoride toothpaste and consumed fluoridated tap water. After 24 months, 89 and 92 children of the test and the control groups were analyzed, respectively. A total of 32 (35.9%) children in the FV group and 43 (46.7%) in the PV group presented new dentine caries lesions (χ² test; p = 0.14), showing relative and absolute risk reductions of 23% (95% CI: -9.5 to 45.9) and 11% (95% CI: -3.5 to 25.0). The mean caries increment differences between the test and control groups were -0.8 (95% CI: -2.0 to 0.4) at the d₂ level and -0.7 (95% CI: -1.9 to 0.4) at the d₃ level. Only 2 minor complaints regarding the intervention were reported. Conclusion: Although safe and well accepted, twice-yearly professional FV application, during 2 years, did not result in a significant decrease in caries incidence.

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