Abstract Objective This study evaluated the clinical efficacy of a new caries detecting dye using a laser fluorescence device (DIAGNOdent). Method Primary and permanent teeth with dentin caries were stained with Caries Check (CC), containing 1% acid red in polypropylene glycol (MW = 300) or Caries Detector (CD), containing 1% acid red in propylene glycol (MW = 76). Primary-CC, primary-CD, permanent-CC and permanent-CD groups were prepared. In the CC groups, stained dentin was completely removed. In the CD groups, pink-stained dentin was retained according to the manufacturers’ instructions. Cavities before and after caries removal were measured with the DIAGNOdent. Data were analyzed using ANOVA and Fisher's PLSD multiple comparison test at α = 0.05. Regression analyses were performed between DIAGNOdent readings and scores obtained from the clinical parameters. Results For all groups, there were no significant differences in the DIAGNOdent readings before treatment. The DIAGNOdent readings after caries removal were: primary-CC (16.0 ± 17.6), primary-CD (9.6 ± 5.2), permanent-CC (11.0 ± 7.0) and permanent-CD (7.1 ± 3.8). Significant differences were identified between the permanent-CC and primary-CD, and permanent-CC and permanent-CD subgroups but not for the primary subgroups. Correlation coefficients between DIAGNOdent readings and clinical parameters were low. Conclusions When dentin stained with Caries Check was completely removed, the DIAGNOdent readings were higher than those recorded when palely-stained pink dentin was retained with the Caries Detector, with significant difference observed for the permanent teeth. Caries Check may be used clinically to avoid excessive removal of caries-affected or sound dentin in permanent teeth but not in primary teeth.