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This study investigated the relationship between caries assessment using a laser fluorescence device (DIAGNOdent), and bacterial invasion in arrested carious dentin detected by polymerase chain reaction (PCR). The ten extracted human molars used in this study had black or dark brown, hard occlusal carious lesions, and were found to be only weakly stained or unstained with a caries detector dye of 1% acid red in propylene glycol. In those extracted human molars, dentin was removed in the direction of the pulp chamber at 150-μm intervals. During each removal (104 sections in total), the dentin surface was assessed with DIAGNOdent, and a dentinal tissue sample was taken with a round bur. Bacterial DNA of each tissue sample was examined using PCR and primers based on the nucleotide sequence of a conserved region of bacterial 16S rDNA. Rates of bacterial detection increased as the DIAGNOdent values increased. When the DIAGNOdent values were <10, the rate of bacterial detection was 0%. The area under the receiver operating characteristic curve of the DIAGNOdent values was 0.87. These results indicate that the DIAGNOdent values of arrested dentinal carious lesion were closely related to the rates of bacterial detection. [ABSTRACT FROM AUTHOR]