This study aimed to evaluate the ability of quantitative light-induced fluorescence (QLF) to assess caries lesion activity using visual examination (VE) as the gold standard. Twenty-four visible white spot lesions on buccal surfaces were examined from 23 children, ages 9 to 14 years. At baseline, the surface was hydrated with water, and thereafter, it was dehydrated with continuous compressed air during image acquisition. QLF images were acquired at 0 (baseline), 5, and 15 s. QLF variables [ QLF V : fluorescence loss ( ? F ), lesion size (S), ? Q : ? F × S ] was recorded. Changes-in- QLF V per second ( ? QLF V ) were determined: ? QLF V = ( QLF VN ? QLF V Baseline ) / N , where N indicates dehydration time. One experienced dentist conducted VE independently using a dental unit’s light, compressed air, and explorer.