Introduction: aerobic fitness is considered one of the most important components of health-related physical fitness, with low levels related to increased risk of premature death from all causes, especially cardiovascular diseases. Objective: to identify the characteristics of adolescents at higher risk of low levels of aerobic fitness. Methods: the study included 696 adolescents (15-17 years of age) enrolled in public high schools of Florianópolis, southern Brazil. This cross-sectional epidemiological study was conducted in Florianópolis, Santa Catarina, Brazil. Aerobic fitness was measured using the modified Canadian Aerobic Fitness Test (mCAFT). Sociodemographic (gender, age, school grade, paternal and maternal schooling, socioeconomic status), and anthropometric variables (body weight, height, triceps and subscapular skinfold thickness), sexual maturation, physical activity, sedentary behavior, and eating habits were collected. Results: it was found that 31.5% of adolescents had low aerobic fitness levels, being higher in boys (49.2%) compared to girls (20.6%). Moreover, girls with sedentary behavior, overweight and high body fat percentage were the groups most likely to have inadequate aerobic fitness. In males, the groups most likely to have inadequate aerobic fitness were those whose parents studied more than eight years, those with low levels of physical activity, and those with inadequate nutrition and excessive body fat. Conclusion: low aerobic fitness levels were present in one third of adolescents and was more prevalent in boys. Lifestyle changes, including replacement of sedentary behaviors by physical and sport activities , may assist in improving the aerobic fitness of Brazilian adolescents.