The origin of cariogenesis in occlusal fissures remains elusive because of limited information about both the fissure structure and the morphologic details of carious lesions occurring there. The present study was aimed at reconstructing the coronal structure of human premolars with the aid of computer-assisted image analysis and, on the basis of the reconstructed structures, investigating the configuration of fissures, their proximity to the enamel-dentin junction (EDJ), and the occurrence and extension of carious lesions around the fissure walls. The coronal portions of the teeth were embedded in polyester resin and then ground off stepwise by hand. Serial images of the enamel contour on successive ground planes were recorded by objective microscopy after it was distinctly demarcated with either 0.5 w/v% carbol fuchsin or 1 w/v% basic fuchsin. Usually, from 150 to 250 photoprints were collected from a single enamel crown to reproduce the details of the fissure structures. The enamel outline (the occlusal and lateral surfaces, fissure walls, and EDJ) and, if present, the outline of caries-attacked areas were traced accurately on those photoprints by means of a digitizing system. The enamel contour data in series were input into an image analyzing system (Nikon COSMOZONE, 2SA). Observation of the computer-drawn coronal enamel allowed us to view the morphologic features of the fissures (shape, divarication, and proximity to EDJ) and to scrutinize the pathway of enamel caries along fissure walls.(ABSTRACT TRUNCATED AT 250 WORDS)