Physical activity, aerobic capacity, muscle strength, muscle endurance and flexibility were measured in 550 boys and 710 girls, 15–19 years of age, randomly selected among adolescents attending school in Denmark. A lower level of physical activity was expected with higher age, based on an earlier report from Sweden. Other types of physical activity than sports decreased with age, but no difference was found between age groups in sport activity, neither in hours per week or percentage of subjects participating in sports. Strength increased in both genders with increasing age and no difference was found in muscle endurance. In boys, aerobic working capacity increased with increasing age, but in relation to body weight, no difference was found between age groups. In girls, aerobic working capacity (1 · min−1) did not differ between age groups, but body weight was higher with higher age. Flexibility increased with age in boys, and did not differ between age groups in girls. Marked differences were found between subjects attending different types of school. High school (gymnasium) students participated more in leisure‐time sport and had higher physical performance than students from trade or vocational schools. Evaluating different levels of sport participation, i. e., 3 levels of competition, sport for health and no sport activity, no systematic difference in fitness was found between the groups of sport participants. A lower fitness level was found in the group of non‐sport participants. No difference was found between age groups in sport activity. The fitness level increased in boys with age and did not change