BACKGROUND: The protective effects of physical activity and fitness on cardiovascular health have clearly been shown among adults and, to a lesser extent, among children and adolescents. However, data are currently lacking pertaining to children and adolescents living in Switzerland. OBJECTIVES: To gather data on the physical fitness and physical/sports activity of children and adolescents aged 9 to 19 years. METHODS: From September 1996 until March 1997, 3540 subjects (1778 girls, 1762 boys) from the canton of Vaud were enrolled in a multifaceted study which included a battery of 7 tests measuring different components of fitness, anthropometric measures and a self-administered questionnaire assessing physical activity, health and lifestyles. RESULTS: Most of the respondents practise sports on a regular basis but boys engage in physical and sports activities much more often than girls: 75% of boys versus 56% of girls spent at least one hour a day in activities inducing sweating, an index of moderate to vigorous physical activity (p < 0.001). Depending on the grade, 56 to 74% of girls and 62 to 88% of boys reported participation in sports clubs (p < 0.01); current participation ranges from 33 to 46% among girls and 64 to 69% among boys (p < 0.001). Participation in physical and sports activities was lower after age 15 than before, and also lower among girls than among boys. As far as fitness is concerned, girls exhibit greater flexibility than boys, while the latter exhibit greater strength and endurance, especially after age 15. Calculated values for the BMI and VO2max are within the ranges published in the international literature for both sexes. CONCLUSION: Programmes and strategies which aim to increase physical activities should be gender-specific and should especially target adolescents aged over 15. Physical/sports activities and fitness could and should be monitored using both a battery of tests and self-administered questionnaires.