DIAGNOdent (KaVo, Biberach, Germany) has shown high diagnostic accuracy and intra-operator agreement for caries detection, both in vitro and in vivo. The aims of this study were to compare DIAGNOdent with visual examination (VI) and bitewing radiographs (BW) for clinical assessment of occlusal fissures, and to evaluate inter-device consistency of clinical recording using four different DIAGNOdent instruments; secondly, to correlate DIAGNOdent readings with microbial culture of the measured site. The subjects were young adults, the material comprising 34 occlusal fissures, scheduled for restorative treatment at the Dental School Clinic of the University of Iceland. Two examiners conducted visual and radiographic assessments. One examiner measured each site with four DIAGNOdent instruments in random order. The fissure was then opened and lesion depth was classified on a 4-point scale. Bacterial samples were taken from the fissure before and after opening. Intra-operator agreement was high (r = 0.85-0.98). Inter-device correlation for the four DIAGNOdent instruments was significant in all cases (r = 0.81-0.92). However, a common cut-off point could not be determined. There was weak but significant correlation between DIAGNOdent readings and all three classes of lesion depth. Level of infection showed very weak correlation with the DIAGNOdent readings. It was concluded that DIAGNOdent is more reliable in detecting dentinal caries if a proper cut-off point is used than in indicating actual lesion depth. Readings from the different instruments were not directly comparable, however, owing to the lack of a common cut-off point.