The purpose of this study was to determine the frequency and severity of dental fluorosis in 5 to 12 and 15 year-old school children in the city of Marinópolis, São Paulo, Brazil, with the purpose of obtaining a baseline data for the monitoring dental fluorosis in this population. All of the school children in the both sexes, in the mentioned ages, enrolled in educational institutions and having the pre-requirement condition of to live in Marinópolis from their birth were involved, totaling 320 school children. The exams were made by one examiner previously calibrated to use the Dean Index. According to the results, the frequency of dental fluorosis in the studied group was 17.2%, however just considering the fluorosis levels that determine aesthetic compromising (mild, moderate and severe), this percentile was 7.19%. The predominant level was the very mild (10.0%) followed for the mild (5.3%), moderate (1.3%) and severe (0.6%). Concluding that the dental fluorosis in the studied population doesn't constitute in a problem with wide dimensions, however, subsequent studies are necessary in the attempt of identifying the causes of moderate and severe fluorosis cases.