The scope of this study was to evaluate the prevalence and severity of dental fluorosis among 11 to 14-year-old schoolchildren in Teresina, Piauí, Brazil, which is a tropical city with a fluoridated public drinking water supply. It involved a cross-sectional observational study on a sample of 571 students in public and private schools. Informed Consent forms were approved for the data collection and the exams were conducted at the schools. Data were recorded on a questionnaire answered by the parents, regarding the socioeconomic and demographic characteristics and oral hygiene habits of the sample. The dental exam was performed qualified dental sugeons. The Thylstrup-Fejerskov (TF) index was used. The prevalence of fluorosis was 77.9%, and only 12.5% of the affected children had TF ≥ grade 3 (with aesthetic damage). The premolars were the teeth most affected by fluorosis. Among the students with the highest severity of fluorosis, 98.6% belonged to the lowest social bracket (> B2), 91.5% were born and had always lived in Teresina, 94.4% consumed water from the fluoridated public supply, 76% used toothpaste for children and 64% of mothers reported that they swallowed toothpaste. The prevalence of fluorosis was high, though the severity was low in individuals exposed to fluoridation since birth.