To analyze the prevalence and symptoms of asthma in students of the Brazilian Amazon municipality of Alta Floresta-MT.Cross-sectional study on the prevalence of asthma in 6 and 7 year-old children and 13 to 14 year-old adolescents, using the Study of Asthma and Allergies in Childhood - ISAAC method, phase I in 2007. Students who answered affirmatively question 2 - "presence of wheezing in the past 12 months" were considered asthmatic.Of the total 2,071 students, 1,072 (51.7%) were children and 999 (48.3%) were teenagers. The prevalence of asthma was 21.4% among schoolchildren, and 12.4% among adolescents (chi2 = 29.29; rho = 0.00). Children presented a higher prevalence than adolescents of the following asthma symptoms: wheezing sometime in life (49.9%), wheezing in the past 12 months (21.4%), 1 to 3 wheezing attacks in the past 12 months (16.4%), and dry cough at night (38.2%). Regarding physician-diagnosed asthma, no difference was observed between the two age groups, with a prevalence of around 6.0%. Male schoolchildren presented a higher prevalence of asthma, physician-diagnosed asthma and four or more wheezing episodes in the past 12 months (rho < 0.05). Male adolescents presented the highest prevalence of strong wheezing affecting speech (rho < 0.05).Alta Floresta presents one of the highest prevalences of asthma in Latin America among schoolchildren in the 6 and 7 year-old age group.