Objective: The aim of this in vivo study was to evaluate the efficacy of low-viscosity light-cured resin infiltration on postorthodonticwhite spot lesions (WSLs) on incipient and advanced lesions using quantitative light-induced fluorescence (QLF).Methods: The study subjects were patients with clinically diagnosed postorthodontic WSLs (n=57). QLF images of the lesions wereobtained using a QLF device (Inspektor-Pro, Amsterdam, The Netherlands) before any treatment. Images were processed using thebuilt-in software (QLF patient v2.0.0.48), which produced fluorescence loss ($Delta F_1$), lesion area (Area1), and impact ($Delta F_1$ x $Area_1$, $Delta Q_1$)values. Lesions were categorized as incipient (−5<$Delta F_1$<−12, n=14) or advanced (−12<$Delta F_1$<−25, n=43). They were then infiltrated withlow-viscosity resin (Icon-DMG, Hamburg, Germany) according to the manufacturer’s instructions. QLF imaging was repeated ($Delta F_2$,$Area_2$, and $Delta Q_2$) from the same aspects assured by the relative software. Kolmogorov-Smirnov, Wilcoxon, and Mann-Whitney testswere used for data evaluation.Results: $Delta F_1$ (−8.40±0.73) and Area1 (3.44±5.19) decreased to −6.58±0.88 and 0.18±0.33 for incipient lesions (p<0.001 and p=0.002,respectively). $Delta F_1$ (−13.20±5.32) and Area1 (4.71±5.56) decreased to −7.51±2.7 and 0.29±1.86 for advanced lesions (p<0.001). WhenΔF, lesion area, and ΔQ changes between the groups were compared, the decrease in ΔF was greater for advanced lesions (p<0.001),whereas the decrease in the lesion area and ΔQ was similar (p=0.690, p=0.291).Conclusions: Infiltration treatment provides improvement of WSLs in terms of fluorescence loss, lesion area, and impact for bothincipient and advanced lesions, with the latter group presenting higher fluorescence loss reduction.