To determine the prevalence of mouth breathing among children aged 3 to 9 years living in the urban districts of the town of Abaeté, MG, Brazil.This study assesses a representative, randomized sample of the town's population (23,596 inhabitants). Children were selected by lots according to a random number table until 370 had been enrolled; this number had been determined by statistical calculation. A protocol for anamnesis and clinical assessment of the patients was specially developed for this project, since no preexisting instruments could be found in the literature that had been validated and were appropriate for the purpose. Data were analyzed using SPSS version 10.5.The prevalence of mouth breathing was found to be 55%, or 204 children.Further studies are needed to validate a questionnaire for the clinical diagnosis of mouth breathers at the primary care level.