This study aimed to evaluate the validity of a portable microhardness testing system (Cariotester) for diagnosis of progression in active caries lesions by comparing data from this device and a laser fluorescence device (DIAGNOdent). Carious dentin in 12 specimens (stained clearly with a caries detector dye) was incrementally removed with a round bur at 150 μm depth intervals from the dentin surface in the direction of the pulp chamber. After each increment (total 138 sites), the Knoop hardness (HK) (evaluated with Cariotester) and DIAGNOdent (D) values were measured. HK values increased as D values decreased (regression formula: HK=0.419+238.342/D (p<0.001); coefficient of determination (R(2)): 0.602). Although DIAGNOdent quantitatively evaluates the degree of caries progression and bacterial infection status in caries lesions, our results demonstrate the validity and convenience of alternatively using microhardness measurements during caries removal to evaluate disease progression.