SUMMARY OBJECTIVE: To evaluate the effectiveness of daily tooth brushing with high-fluoride toothpaste on white spot lesion (WSL) formation in adolescents during treatment with fixed orthodontic appliances (FOA). MATERIALS AND METHODS: Four hundred and twenty-four healthy 11- to 16-year-old patients, referred to five Orthodontic Specialist Clinics, were randomized to use either toothpaste containing 5000 ppm fluoride or regular toothpaste with 1450 ppm fluoride. To be eligible for inclusion, the patients had to be scheduled for bimaxillary treatment with FOA for an expected duration of at least 1 year. The primary and secondary outcome measures were prevalence and incidence of WSL, as registered from digital photos of the maxillary incisors, canines, and premolars taken before onset and immediately after debonding. The photos were evaluated separately by two blinded and calibrated clinicians using a 4-step score. A random sample of 50 cases was reassessed to check intra- and interexaminer reliability (Kappa = 0.70; 0.74). RESULTS: The use of high-fluoride toothpaste resulted in fewer WSL (P = 0.042) with a prevented fraction of 32%. The lateral incisor was most commonly affected in both groups. CONCLUSION: To prevent WSL during treatment of FOA, daily use of high-fluoride toothpaste may be recommended.