Fluoridated water is still considered an effective caries-preventive method in Public Health, and at the present time it has been the object of studies with a focus on the prevalence of dental fluorosis. Objectives: The aim of this study was to conduct an epidemiological survey and to evaluate the prevalence of dental caries and fluorosis in 35 cities in the State of São Paulo, Brazil, relative to the presence or absence of fluoridated water, in the year 2002. Methods: We examined 5969 schoolchildren (12 years of age), 3064 female and 2905 male. We selected 35 cities in the State of São Paulo, 80% of which had fluoridated water supplies. The clinical teams constisted of 5 examiners from cities previously calibrated (kappa > 0.82). The clinical examination was conducted in an area with natural light, with “ball point” probes and a plain buccal mirror. The criteria used for DMFT and fluorosis indices were those recommended by the 4th edition of the manual from the World Health Organization (1997). Results: The results showed that the DMFT index was 2.34 in the cities with fluoridated water and 3.51 in the cities without fluoridated water; the DMFT index was 2.52 in the total sample. Participants from cities with fluoridated water represented 13.77% of those with fluorosis and 82.27% without fluorosis. In the cities without fluoridated water, 0.66% of the participants presented with fluorosis, and 98.58% did not. Conclusion: Fluoridated water is associated with the history of caries and dental fluorosis in 12-year-old schoolchildren in the State of São Paulo, Brazil.