PREVALENCE OF SEDENTARY LIFESTYLE AND ASSOCIATED FACTORSIN BRAZILIAN ADOLESCENTSABSTRACTPurpose: To describe levels of physical activity in adolescents (10-19 years) living in Southern Brazil, and to explore the association between sedentary lifestyle and demographic, economic, health-related, and parental variables. Methods: Population-based cross-sectional study including 857 adolescents selected through a multistage random sampling strategy. Sedentary lifestyle was defined as 300 min/wk of moderate to vigorous physical activity, following current recommendations for adolescents. Physical activity data collected by questionnaire were compared with pedometer counts and with a longer version of the questionnaire in a sub-sample of 92 adolescents. A cutoff point of 10,000 steps per day was used for classifying adolescents as sedentary or not by pedometers.Results: Reliability of the questionnaire was good, and its validity in comparison to a longer questionnaire was also satisfactory. Although the sensitivity of the questionnaire for detecting sedentary lifestyle in comparison to pedometers was good (77.1%), its specificity was low (42.6%). The prevalence of sedentary lifestyle by questionnaire in the whole sample was 69.8% (CI95% 66.7; 72.9). Boys were more active than girls. The prevalence of sedentary lifestyle increased with age in girls, but not in boys. Among boys, those from low economic levels were more likely to be sedentary. Among girls, paternal physical activity was directly associated with adolescent activity levels.Conclusion: Urgent strategies aimed at increasing levels of activity of adolescents are necessary in Brazil given the high prevalence of sedentary lifestyle detected in this study. The variables associated with sedentary lifestyle in adolescence varied between boys and girls.Key-words: sedentary lifestyle, exercise, adolescent, epidemiology, Brazil.