Introduction: Orthodontic treatment with fixed elements increases the risk of developing white spot lesions due to additional retention opportunities for biofilm. One approach for the esthetic treatment of these lesions is caries infiltration. In order to estimate the final masking effect of the resin infiltration, re-wetting with water or ethanol is often performed just after the etching procedure. Patient cases: Four patients who were diagnosed with white spot lesions in the esthetically visible area during treatment with fixed orthodontic appliances were informed about the possibility of optically masking these lesions using resin infiltration. The infiltration (Icon, DMG) was performed according to the manufacturer's instructions, but the etching procedure was performed up to three times. The decision regarding whether to repeat the etching procedure was made subjectively by the practitioner during the re-wetting test with ethanol. The masking effect generated by ethanol and resin infiltration was analyzed using digital photographs before treatment, before and during the first 9 sec of re-wetting as well as one week after treatment. Result: In all 4 patient cases, the observed minimum color difference during re-wetting appears to be a good indicator for assessing whether a satisfactory masking effect can be achieved. Furthermore, in the total of 55 treated white spot lesions, lesions with higher color value differences were etched more frequently than lesions with less pronounced color value differences. Overall, the masking effect one week after resin infiltration was satisfactory in all cases. Discussion: During the re-wetting test, the minimum color difference between the initial lesion and the intact enamel which surrounds the lesion seems to be a good predictor of the final outcome of resin infiltration of post-orthodontic white spot lesions. Furthermore, white spot lesions with a higher initial color difference appear to require multiple etching procedures. Conclusion: Resin infiltration is a useful microinvasive method for masking white spot lesions which develop during orthodontic treatment with fixed appliances. Moreover, the temporary masking effect achieved by the re-wetting test with ethanol can be used as an indicator for estimating the number of etching procedures required. Keywords: caries infiltration, enamel lesion, esthetics, post-orthodontic treatment, white spot lesion