An in vivo study was conducted to verify the ability of laser fluorescence (LF) to assess the activity status of occlusal caries in primary teeth, using different air-drying times. Occlusal sites (707) were examined using LF (DIAGNOdent) after air-drying for 3 s and 15 s, and the difference between readings (DIF15 s-3 s) was calculated. For concurrent validation of LF, visual criteria-Nyvad (NY) and Lesion Activity Assessment associated with the International Caries Detection and Assessment System (LAA-ICDAS)-were the reference standards for lesion activity. Histological exam using a pH-indicator dye (0.1% methyl red) was performed in 46 exfoliated/extracted teeth for criterion validation. LF readings and DIF15 s-3 s were compared using Kruskall-Wallis and Mann-Whitney tests. Receiver operating characteristic analyses were performed and validity parameters calculated, considering the caries activity assessment. Using NY, active lesions (3 s: 30.0+/-29.3; 15 s: 34.2+/-30.6) presented higher LF readings than inactive lesions (3 s: 17.0+/-16.3; 15 s: 19.2+/-17.3; p<0.05), different from LAA-ICDAS. Active cavitated caries resulted in higher LF readings (3 s: 50.3+/-3.5; 15 s: 54.7+/-30.2) than inactive cavitated caries (3 s: 19.9+/-16.3; 15 s: 22.8+/-16.8). Therefore, LF can distinguish cavitated active and inactive lesions classified by NY, but not by LAA-ICDAS; however, this difference might be related to the visual system rather than to LF. The air-drying time could be an alternative to improve the caries activity assessment; however, longer air-drying time is suggested to be tested subsequently.