In a retrospective survey the mean number of intact proximal surfaces on the first molars of 13-year-old children was found to increase from 2.6 to 4.8 following 7 years of a preventive program with fortnightly fluoride mouthrinsings (0.2% NaF). This improvement was significant and corresponded to a reduction of the totally filled surfaces from 30.0 to 16.6. Caries on the proximal surfaces on the first molars was assessed from bite-wing radiographs. Judged from interexaminer comparisons, the number of filled surfaces was a reliable parameter of the caries prevalence in 13-year-old children. In a group of 54 children aged 13 who participated in the preventive program, the caries experience on the proximal surfaces of the first molars was significantly associated with the total DMFS.