BACKGROUND There is evidence that the prevalence of dental fluorosis in many countries has increased over the last three decades along with a noted decrease of dental caries. To date no comparative studies of dental caries and dental fluorosis have been conducted in Bulgaria reflecting the present-day conditions of life. OBJECTIVE To study comparatively the prevalence of dental fluorosis and dental caries and to establish what relationship, if any, there is between them. METHODS We performed a comparative analysis of three epidemiological studies carried out in Plovdiv and Dimitrovgrad in 2004, 2005 and 2008. The hypothesis tested was that dental caries was less frequent in populations with higher prevalence of dental fluorosis (with predominance of mild fluorosis cases). Dental fluorosis was diagnosed using Dean's criteria, and dental caries was scored using the WHO criteria. The results were analysed using the alternative analysis and analysis of variance at a level of significance P < 0.05. RESULTS The prevalence of dental fluorosis in the studied populations in Dimitrovgrad in 2004 was 56.99%, in Plovdiv in 2005--7.80%, and in 2008--23.18%. The comparison of the studies in Dimitrovgrad and Plovdiv in 2005 showed lower prevalence of dental caries and lower DMFT values in all age groups (P < 0.001) in Dimitrovgrad, where the prevalence of dental fluorosis was greater than that in Plovdiv (P < 0.001). The comparison between the studies in Plovdiv alone in 2005 and 2008 showed an increase in dental fluorosis in 2008 (P < 0.001). The prevalence of dental caries was higher in almost all groups, but the differences failed to reach statistical significance (P < 0.001). The DMFT index was higher in 2008 (P < 0.001) with the exception of some age groups. CONCLUSIONS The higher prevalence of dental fluorosis does not necessarily lead to low prevalence of dental caries and low DMFT values in the specific population.