Objective To analyse the secular trends in aerobic fitness performance and some of its determinants (body mass index (BMI) and leisure time physical activity (LTPA)) in adolescents.

Design Cross-sectional population-based studies in 1976 and 2001 in Finland. A stratified random sample of Finnish 13–18-year-old adolescents was studied in 1976 (n = 717; 384 boys and 333 girls) and in 2001 (n = 558; 305 boys and 253 girls). The main outcome measure was aerobic fitness, estimated with a 2000 m (for boys) and 1500 m (for girls) running test; the weight and height of participants were also measured. Self-reported weekly frequency of LTPA of at least 30 min duration and regularity of participation in organised sport were obtained by questionnaire. Identical methods were used in 1976 and 2001.

Results Running time was longer in 2001 compared to 1976 in boys (56 s, Cohen d = 0.46, medium effect size; p<0.001) and girls (29 s, d = 0.32, small; p<0.01). BMI and participation in LTPA explained more of the variance in aerobic fitness in 2001 than in 1976 in boys and girls.

Conclusions Aerobic fitness of school-aged children deteriorated between the measurement points. BMI and organised LTPA were better associated with aerobic fitness in 2001 than in 1976. An increase in overweight and obesity was associated with the decrease in aerobic fitness. Although the importance of organised LTPA to fitness increased, it is possible that the decrease in overall physical activity between 1976 and 2001 contributed most to the decrease in the level of aerobic fitness.