Epidemiological data reveal that the prevalence of dental caries in western countries has decreased in recent decades. The aim of this study was to investigate how dentists and dental hygienists assess dental caries lesions in bite-wing radiographs between 1983 and 2003. All dentists and dental hygienists in Public Dental Health in Uppsala County were offered to take part in the study. The participants assessed manifest and initial caries lesions in eight bite-wing radiographs from three patients individually. An X-ray viewer and binoculars were used. The assessments were repeated in the same radiographs every five years, a total of five times, between 1983 and 2003. In the different test occasions 80-103 dentists and 11-48 dental hygienists participated. The registration of dental caries changed between 1983 and 2003. The number of manifest lesions registered by dentists decreased between 1983 and 1988, but were stable after 1988. Dental hygienists showed no changes in the registration of manifest lesions during the study. Initial lesions registered by dentists and dental hygienists increased between 1988 and 1998. Assessments of initial caries lesions displayed a wider range than manifest lesions. Increasing age and more years in the profession resulted in fewer registered initial caries lesions. Dental hygienists had a tendency to register less caries than dentists. In conclusion, the result of the study indicate that inclusion of initial caries lesions in epidemiological reports should lead to a reduction in reliability. The changes in assessments of manifest caries lesions that took place in the 19805s should be considered when epidemiological data are evaluated.