A systematic review was performed of published data on the caries-inhibiting effect of preventive measures during orthodontic treatment with fixed appliances. The purpose was to develop evidence-based recommendations about the most effective means of preventing white spot lesions in orthodontic patients. The 15 studies included were divided into four groups based on comparable preventive measures: fluoride, chlorhexidine, sealants and bonding materials. The caries-inhibiting effect of the preventive measures was expressed by the prevented fraction (PF). The overall PF of the fluoride-releasing bonding materials was 20% (SEM 0.09). This effect was, however, not statistically significant. It was impossible to calculate an overall PF for the other preventive measures, but the tendency of their caries-inhibiting effect has been described. The use of toothpaste and gel with a high fluoride concentration of 1,500-5,000 ppm or of complementary chlorhexidine during orthodontic treatment showed a demineralisation-inhibiting tendency. The use of a polymeric tooth coating on the tooth surface around the brackets showed almost no demineralisation-inhibiting effect. Many publications had to be excluded from this systematic review because of improper research designs. Future clinical trials are needed to give evidence- based advice on the optimal caries-prevention strategy.