BACKGROUND White spot lesions that form during orthodontic treatment are a problem for patients and clinicians. Lesion infiltration with low-viscosity light-cured resin has been proposed as a treatment to inhibit further demineralization. The purpose of this study was to assess the durability of assimilation of white spot lesions and sound adjacent enamel achieved over 6 months with resin infiltration. METHODS Twenty-one consecutive subjects with 231 noncavitated, unrestored white spot lesions after multibracket treatment were recruited at the Department of Orthodontics, University of Göttingen (Germany), for lesion infiltration. A simple randomized, split-mouth, controlled design was used to allocate subjects to the treatment and control groups. In the treatment group, white spot lesion infiltration of the anterior teeth was performed with low-viscosity light-cured resin after enamel conditioning with a 15% HCl gel. Color and lightness of the white spot lesions and the sound adjacent enamel were assessed with a spectrophotometer before infiltration and after 1 day, 1 week, 4 weeks, 3 months, and 6 months, using the system of the Commission Internationale de l'Eclairage. Multifactorial analysis of variance with repeated measures and pair-wise comparisons were used to analyze the effects of infiltration and time elapsed on the color differences at an α level of 5% and a power of 80%. RESULTS Analysis of 20 subjects and 39 quadrants in each group (108 teeth in the control group; 111 teeth in the treatment group) showed that both parameters of treatment and time duration had globally a highly significant influence on the color difference values. Assimilation of white spot lesion color to the surrounding enamel after infiltration was stable with no significant changes over 6 months; the mean color difference of white spot lesions vs sound adjacent enamel (ΔE baseline vs 6 months) was 2.55 (95% confidence interval [CI], 1.431-3.678). The untreated control teeth showed no significant changes over 6 months compared with the baseline: mean (ΔE), 0.29 (95% CI, -0.335-0.928). No important adverse events or side effects were observed. CONCLUSIONS Resin infiltration improves the esthetic appearance of demineralized teeth. The results showed sufficient durability over 6 months.