OBJECTIVE:To assess the association between daily exposure to air pollution and lung function in school children. METHODS:Panel study with a random sample of 118 students (between 6 and 15 years of age), enrolled in a public school of the city of Rio de Janeiro, state of Rio de Janeiro, and living within 2 km of the study site. Data on students' characteristics were obtained with a questionnaire, including the International Study of Asthma and Allergies in Childhood -ISAAC. Daily peak expiratory fl ow measurements were taken to measure lung function. Daily data on PM10, SO 2 , O 3 , NO 2 and CO levels, temperature and humidity were provided by a portable monitor. Repeated measurements of lung function were associated with pollutant levels with a multilevel model adjusted for time trend, temperature, air humidity, exposure to smoking at home, presence of asthma, height, sex, weight and age of children. RESULTS:Mean peak expiratory fl ow was 243.5 l/m (sd=58.9). The lowest mean peak expiratory fl ow was 124 l/m, and the highest, 450 l/m. For the 10 μg/m³ increase in PM10, there was a 0.34 l/min decrease in mean peak fl ow on the third day. For the 10 μg/m³ increase in NO 2 , there was a decrease between 0.23 l/min and 0.28 l/min in mean peak fl ow after exposure. CO and SO 2 effects on students' peak fl ow were not statistically signifi cant. O 3 showed a protective result: an increase in 10 μg/m³ of O 3 would be associated, after a day of exposure, with a 0.2 l/min increase in mean lung function. CONCLUSIONS:Even within acceptable levels most of the time, air pollution, especially PM10 and NO 2 , was associated with a decrease in lung function in children living in the city of Rio de Janeiro.