Objective: To evaluate criteria for the diagnosis of asthma in an epidemiological survey. Methods: Adolescents (13-14 years of age) and legal guardians of schoolchildren (6-7 years of age) in the city of Santo André, Brazil, completed the International Study of Asthma and Allergies in Childhood (ISAAC) standard written questionnaire. Affirmative responses regarding wheezing within the last 12 months, asthma ever, bronchitis ever (question added at the end of the questionnaire), as well as the overall ISAAC score above the predefined cutoff points, were considered indicative of asthma. Results: The legal guardians of 2,180 schoolchildren and 3,231 adolescents completed the questionnaires properly. Depending on the criterion adopted, the prevalence of asthma ranged from 4.9% to 26.8% for the schoolchildren and from 8.9% to 27.9% for the adolescents. The criteria with the lowest and highest prevalences were, respectively, physician-diagnosed asthma and physician-diagnosed bronchitis. When compared with other criteria, physician-diagnosed bronchitis showed concordance levels between 71.9% and 79.4%, positive predictive values between 0.16 and 0.63 and poor concordance (kappa: 0.21-0.46). Strong concordance levels were found only between wheezing within the last 12 months and the overall ISAAC score (kappa: 0.82 and 0.98). Conclusions: The prevalence of asthma varied significantly, depending on the criterion adopted, and there was poor concordance among the criteria. Wheezing within the last 12 months and the overall ISAAC score are the best criteria for the diagnosis of asthma, whereas the question regarding bronchitis ever did not improve the questionnaire. Modifications in this instrument can make it difficult to draw comparisons and should therefore be carefully evaluated.

Keywords: Asthma; Bronchitis; Diagnosis; Epidemiology; Child; Adolescent.