ABSTRACT Introduction: Physical activity (PA) remains the most important modifiable risk factor in the prevention of chronic diseases that are major killers in the modern era. However, many young people today do not meet the recommended guidelines on PA. Few studies were found on the levels of physical activity among adolescents from transitional countries. Objective: To determine the levels of physical activity in a representative sample of healthy adolescents from Pančevo (Serbia), and the factors that determine these level. Methods: This cross-sectional study included 401 randomly selected adolescents (191 boys and 210 girls) from the city of Pančevo, Serbia. The long version of the International Physical Activity Questionnaire (IPAQ), validated for Serbia, was applied. PA was expressed as metabolic equivalent-minute per week (MET-min/week) and classified as low, moderate, or high. Results: The median PA for the whole sample was 2049.18 MET-min/week. The average PA score in boys was higher (2680 MET-min/week) than in girls (1479 MET-min/week). The prevalence of respondents with inadequate PA was 53.9%, and was higher in girls than in boys. Boys showed the higher values for intense and moderate PA, while the prevalent type of PA among girls was walking. Level of PA did not depend on the adolescent's nutritional status. The most important predictors of PA among the participants were sex, father's PA, and hours spent on sedentary activities like using a computer and watching television. Conclusion: Adolescents in Pančevo do not practice enough physical activity, especially girls. Better strategies are needed, to increase levels of PA, considering the main predictors in the adolescent population. Level of Evidence II; Retrospective study.