Objective: To define the reproducibility and accuracy of the Prepometer device (Hager & Werken) and to correlate the measured electrical resistance with the remaining dentin thickness. Method and Materials: In vivo standardized cavities were made on the buccal side in the cervical region of 12 intact teeth. Measurements with the Prepometer were completed before tooth extraction. After extraction, in vitro measurements of the remaining dentin thickness were performed using a µfocus-computerized tomography scanner. These in vitro scan data were compared with the in vivo measurements, and correlation coefficients calculated. The intra- and interinvestigator variability were defined, using a paired 2-tailed Student t test. The level of significance was set at 5%. Results: Although both the intra- and interinvestigator variability scored well, there was no clear correlation between the Prepometer measurements and the remaining dentin thickness. Because the device determines the electrical resistance of the dentin, which is related to dentin permeability, real remaining dentin thickness cannot be measured. A reasonable safety zone (3 of 12 teeth scored false negative and 1 of 12 teeth scored false positive) in terms of remaining dentin thickness was observed. Conclusions: The Prepometer, a device developed to screen dentin-surface permeability by measuring its electrical resistance, was shown to be reproducible in its use. However, electrical resistance was shown not to correlate with dentin thickness. Although the Prepometer might help to estimate the risk of pulp injury after tooth preparation, its effectiveness still remains to be proven in a randomized clinical trial. Keywords: dentin permeability, dentin thickness, microcomputerized tomography, Prepometer, pulp injury, tooth preparation