Written questionnaires (WQ) have been widely used in epidemiologic studies. In order to yield comparable results, they must be validated after translation to another language. The International Study of Asthma and Allergies in Childhood (ISAAC) WQ has been previously validated by a comprehensive study, but its validation in Brazil has not been performed. Our objectives were to validate the rhinitis component of the ISAAC's self-applicable WQ following its translation to Portuguese, and to determine the prevalence of rhinitis and related symptoms among Brazilian children living in the city of São Paulo. A group of 10 pediatricians and 10 pediatric allergists graded the questions from 0 to 2 and established a maximum score for each question. The WQ was answered by parents or guardians of children 6–7 years of age with rhinitis (R) (n = 27) and of control children of the same age without rhinitis (C) (n = 27). The WQ was also completed by adolescents 13–14 years of age with rhinitis (R) (n = 32) and without rhinitis (C) (n = 32). Half of these individuals answered the same WQ after 2–4 weeks, to ensure reproducibility. Cut-off scores of 4 and 3 were identified for the 6–7- and 13–14-year-old groups, respectively, as scores predictive of rhinitis. The prevalence of rhinitis was 28.8% in the group of 3005 children 6–7 years of age and 31.7% in the group of 3008 children 13–14 years of age, respectively. Using the global cut-off score, these prevalences were even higher, in the order of 34.7% and 40.7%, respectively. In conclusion, the rhinitis component of the ISAAC WQ was proven to be reproducible, adequate and able to discriminate children and adolescents with and without rhinitis, and revealed that the prevalence of rhinitis among Brazilian children living in the city of São Paulo was as high as the prevalence of rhinitis in other areas of the world.