OBJECTIVE The development of white spot lesions around orthodontic brackets and gingivitis is a common problem during orthodontic treatment with fixed appliances. This prospective randomized double-blind controlled clinical trial investigated the preventive efficacy of a one-time application of two commonly used fluoride varnishes in patients with low to moderate caries risk. METHODS Ninety adolescent orthodontic patients with a low to moderate caries risk were prospectively randomized to three groups of 30 patients each: (1) standardized dental hygiene with fluoride toothpaste and one-time application of placebo varnish (control) or (2) of elmex® fluid or (3) of Fluor Protector S on all dental surfaces at the start of fixed therapy. The extent of enamel demineralization and gingivitis was determined with the ICDAS and the gingivitis index (GI) at baseline and after 4, 12, and 20 weeks. RESULTS Each treatment group showed a significant increase of the ICDAS index, but not of the GI over the course of time with no significant intergroup differences detectable. CONCLUSIONS A one-time application of fluoride varnish at the start of orthodontic treatment did not provide any additional preventive advantage over sufficient dental hygiene with fluoride toothpaste with regard to formation of white spots and gingivitis in patients with a low to moderate caries risk. CONCLUSIONS In dental practice, patients often receive an application of fluoride varnish at the start of orthodontic treatment with fixed appliances. However, the efficacy of this procedure is still unclear.