The present study described current physical activity, determined compliance with physical activity guidelines and assessed aerobic fitness in a nationally representative sample of 9- and 15-year-olds in Norway. In 2005-2006, 2299 children and adolescents were randomly recruited. The participation rate was 89% and 74% among the 9- and 15-year-olds, respectively. Physical activity was assessed objectively by accelerometry, and aerobic fitness was measured directly as peak oxygen uptake during a cycle ergometry test. Boys were more physically active than girls, and 9-year-olds were substantially more active than 15-year-olds. Physical activity was higher during weekdays than weekends, and 9-year-olds were most active during spring. While four out of five children met current physical activity guidelines, only half of the adolescents did. The mean (SD) values for peak VO(2) were: 9-year-old boys, 48.2 (7.1) mL/min/kg; 9-year-old girls, 42.9 (6.7) mL/min/kg; and 15-year-old girls 41.1 (6.0) mL/min/kg and 15-year-old boys 51.9 (8.0) mL/min/kg. Because of the high participation rate, this study provides a good description of the physical activity and aerobic fitness in the young population. Finally, girls and adolescents seem appropriate targets when promoting physical activity in order to increase the proportion meeting the recommendations.