A total of 1,671 children aged 11, 13 and 15 years from a nationwide random sample of schools completed an anonymous questionnaire on their social backgrounds, social networks, health, life satisfaction, and health behaviour. The proportion practising outdoor activities at least three times a week was 57%; 37% did some physical exercise in their leisure time sufficient to make them breathless or sweat at least four times a week; 43% at least four hours a week. Two thirds were active members of a sports club. A minority of 8% did not carry out any physical exercise during their leisure time sufficient to make them breathless or sweat. Boys did more physical exercise than girls irrespective of the measurement of exercise. The most important motives for physical exercise were to improve health, to get in good shape, to have fun, and to make new friends. More than 90% of the children intended to do physical exercise at the age of 20. High intensity of physical activity was associated with high social class, strong social integration in the peer group, easy contact with parents and friends, physical exercise among parents and best friends, high degree of life satisfaction, and good health.