The aim of this study was to evaluate the effect of professional flossing with NaF and SnF2 gels on caries development on approximal tooth surfaces. Two-hundred-and-eighty 13-year-old schoolchildren were divided into 3 groups: (1) NaF (n = 97), (2) SnF2 (n = 85), and (3) placebo gel group (n = 98). The investigation was carried out double-blind. The children were treated 4 times a year for 3 years with 1% NaF gel, 1% SnF2 gel, or placebo gel. The treatment was carried out by dental nurses and the time required per visit was approximately 10 min. After 3 years, the mean approximal caries increment, including initial caries lesions, was 2.8 in the NaF, 2.4 in the SnF2, and 4.0 in the placebo gel group (P< 0.05 for SnF2 vs placebo); a reduction compared to the placebo of 30% and 39% in the NaF and SnF2 groups, respectively. Thus, professional flossing with NaF or SnF2 gel carried out 4 times a year may be considered as an interesting caries-preventing method for large-scale application in schoolchildren.