OBJECTIVE The decalcification of enamel is a serious clinical problem in orthodontic patients and is usually observed as white spot lesions surrounding brackets. This study's purpose was to evaluate the effect of professionally applied 1.23 percent fluoride foam on reducing the formation of white spot lesions (WSLs) in patients with fixed orthodontic appliances. METHODS In a randomized, double-blind, placebo-controlled trial, 100 participants were randomly divided into two groups. The two groups received fluoride foam and placebo foam, respectively, every two months during the treatment. The examinations before bonding and after debonding were performed by one examiner and included the presence and severity of WSLs on incisors, canines, and premolars. RESULTS The incidence of WSLs was approximately 13 percent in the fluoride foam group and 51 percent in the placebo group (P<.001). The mean increment of WSLs score was significantly lower in the fluoride foam group (0.71 ± 2.80) than in the placebo group (4.36 ± 5.41; P<.001). The preventive fraction was approximately 76 percent, and the number needed to treat was calculated as 2.6. CONCLUSIONS Professional application of 1.23 percent fluoride foam during orthodontic treatment effectively reduced the development of white spot lesions. A prophylactic regimen based on the routine use of fluoride foam during orthodontic treatment is recommended.