The aim of this paper was to quantitatively assess caries changes of teeth by using digital image analysis. Digital images of stained sections of crowns of teeth were acquired with a computer-assisted light microscope. In each image, spots representing the main and total demineralization of enamel were segmented to determine their area. The area of total demineralization was significantly different between premolars with sealed fissures and unprotected premolars as indicated by the Mann–Whitney test. Fissures on occlusal surfaces of premolars were characterized by their width, height, and distance from the bottom of the fissure to the enamel–dentin junction. This distance was also significantly different between protected and unprotected teeth. The results of our in vitro studies of enamel lesions allow us to plan an effective diagnosis, prophylaxis, and treatment of early caries changes in in vivo conditions. © 2003 Biomedical Engineering Society.