This triple blinded RCT two arms study determines the efficacy of fluoride rinse (0.32% NaF-solution) for a period of 6 and 12 months (6m,12m) on the caries increment, plaque occurrence and gingival conditions in 10-15 year old Danes undergoing orthodontic treatment. Estimations showed that 24 subjects were required in the Test Group (TG) as well as in the Control Group (CG) at the end of the study. A total of 61 subjects were randomly assigned to TG (n=30) and the CG(n=31). Participants rinsed twice a week with 10 ml NaF solution (TG) or 10 ml of a placebo solution (CG). One assessor recorded at baseline, after 6-and 12 months caries (ICDAS index) and plaque and gingival status (Löe index). Four participants in each group dropped out during the study period. At patient level 27% in TG and 59% in CG showed caries progression during 12 month (p<0.05). Parametric tests confirmed that CG-participants had significant higher progression rate than TG-participants from 0m to 12m (p=0.014). The preventive fraction was 77% in favour of the TG-participants. Similar calculations from baseline to 6 months after, there were no significant caries increment difference between TG and CG. At 0m and 12m there was no significant difference in the plaque index and the gingival index between the two groups (p-values>0.05). The study conclusion was that rinsing twice a week with a 0.32% NaF solution during orthodontic treatment with fixed appliance had a significant effect on caries progression in children and adolescents during a treatment period of 12 months.